content/static/html/helpers: add anchors for section ids

Package pages are missing the paragraph symbol / anchor
for some items.

Add this symbol / anchor to allow linking to the tags
that have the "section-sourcefiles", "section-directories",
"section-readme", "section-documentation" ids.

Fixes golang/go#45210

Change-Id: I37538bcb30dbaa1cca335119775ff56b54c084fb
GitHub-Last-Rev: e058e6f11d3b84ab0947866f609bcfa7fa43b21a
GitHub-Pull-Request: golang/pkgsite#24
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/304649
Reviewed-by: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>
Trust: Julie Qiu <julie@golang.org>
diff --git a/content/static/css/unit_directories.css b/content/static/css/unit_directories.css
index 15cff97..00f03b9 100644
--- a/content/static/css/unit_directories.css
+++ b/content/static/css/unit_directories.css
@@ -7,6 +7,14 @@
 .UnitDirectories {
   margin-bottom: 2rem;
 }
+.UnitDirectories h2 a.UnitDirectories-idLink,
+.UnitDirectories summary a {
+  opacity: 0;
+}
+.UnitDirectories h2:hover a,
+.UnitDirectories summary:focus a {
+  opacity: 1;
+}
 .UnitDirectories-title {
   border-bottom: 0.0625rem solid var(--gray-8);
   font-size: 1.375rem;
diff --git a/content/static/css/unit_doc.css b/content/static/css/unit_doc.css
index 487d779..6b421a8 100644
--- a/content/static/css/unit_doc.css
+++ b/content/static/css/unit_doc.css
@@ -11,6 +11,14 @@
   margin-bottom: 2rem;
   word-break: break-word;
 }
+.UnitDoc h2 a.UnitDoc-idLink,
+.UnitDoc summary a {
+  opacity: 0;
+}
+.UnitDoc h2:hover a,
+.UnitDoc summary:focus a {
+  opacity: 1;
+}
 .UnitDoc-title {
   border-bottom: 0.0625rem solid var(--gray-8);
   font-size: 1.375rem;
diff --git a/content/static/css/unit_files.css b/content/static/css/unit_files.css
index 192299b..0cb5ec8 100644
--- a/content/static/css/unit_files.css
+++ b/content/static/css/unit_files.css
@@ -7,6 +7,14 @@
 .UnitFiles {
   margin-bottom: 2rem;
 }
+.UnitFiles h2 a.UnitFiles-idLink,
+.UnitFiles summary a {
+  opacity: 0;
+}
+.UnitFiles h2:hover a,
+.UnitFiles summary:focus a {
+  opacity: 1;
+}
 .UnitFiles-title {
   border-bottom: 0.0625rem solid var(--gray-8);
   font-size: 1.375rem;
diff --git a/content/static/css/unit_readme.css b/content/static/css/unit_readme.css
index f9c437e..f98656f 100644
--- a/content/static/css/unit_readme.css
+++ b/content/static/css/unit_readme.css
@@ -7,6 +7,14 @@
 .UnitReadme {
   margin-bottom: 2rem;
 }
+.UnitReadme h2 a.UnitReadme-idLink,
+.UnitReadme summary a {
+  opacity: 0;
+}
+.UnitReadme h2:hover a,
+.UnitReadme summary:focus a {
+  opacity: 1;
+}
 .UnitReadme-title {
   border-bottom: 0.0625rem solid var(--gray-8);
   font-size: 1.375rem;
diff --git a/content/static/html/helpers/_unit_directories.tmpl b/content/static/html/helpers/_unit_directories.tmpl
index 3047e7b..8841db4 100644
--- a/content/static/html/helpers/_unit_directories.tmpl
+++ b/content/static/html/helpers/_unit_directories.tmpl
@@ -8,6 +8,7 @@
   <div class="UnitDirectories js-unitDirectories">
     <h2 class="UnitDirectories-title" id="section-directories">
       <img height="25px" width="20px" src="/static/img/pkg-icon-folder_20x16.svg" alt="">Directories
+      <a class="UnitDirectories-idLink" href="#section-directories">¶</a>
     </h2>
     <div class="UnitDirectories-expandButton js-expandAllDirectories">
       <button>Expand all</button>
diff --git a/content/static/html/helpers/_unit_doc.tmpl b/content/static/html/helpers/_unit_doc.tmpl
index 9029f2c..0a13027 100644
--- a/content/static/html/helpers/_unit_doc.tmpl
+++ b/content/static/html/helpers/_unit_doc.tmpl
@@ -8,6 +8,7 @@
   <div class="UnitDoc">
     <h2 class="UnitDoc-title" id="section-documentation">
       <img height="25px" width="20px" src="/static/img/pkg-icon-doc_20x12.svg" alt="">Documentation
+      <a class="UnitDoc-idLink" href="#section-documentation">¶</a>
     </h2>
     {{template "unit_build_context" .}}
     <div class="Documentation js-documentation">
diff --git a/content/static/html/helpers/_unit_files.tmpl b/content/static/html/helpers/_unit_files.tmpl
index 568c06f..48e1c38 100644
--- a/content/static/html/helpers/_unit_files.tmpl
+++ b/content/static/html/helpers/_unit_files.tmpl
@@ -8,6 +8,7 @@
   <div class="UnitFiles js-unitFiles">
     <h2 class="UnitFiles-title" id="section-sourcefiles">
       <img height="16px" width="12px" src="/static/img/pkg-icon-file_16x12.svg" alt="">Source Files
+      <a class="UnitFiles-idLink" href="#section-sourcefiles">¶</a>
     </h2>
     <div class="UnitFiles-titleLink">
       <a href="{{.SourceURL}}" target="_blank" rel="noopener">View all</a>
diff --git a/content/static/html/helpers/_unit_readme.tmpl b/content/static/html/helpers/_unit_readme.tmpl
index 07f209f..546bd20 100644
--- a/content/static/html/helpers/_unit_readme.tmpl
+++ b/content/static/html/helpers/_unit_readme.tmpl
@@ -8,6 +8,7 @@
   <div class="UnitReadme {{if .ExpandReadme}}UnitReadme--expanded{{end}} js-readme">
     <h2 class="UnitReadme-title" id="section-readme">
       <img height="25px" width="20px" src="/static/img/pkg-icon-readme_20x16.svg" alt="">README
+      <a class="UnitReadme-idLink" href="#section-readme">¶</a>
     </h2>
     {{if .Readme.String }}
       <div class="UnitReadme-content" data-test-id="Unit-readmeContent">