_content/doc/tutorial/json: add encoding/json/v2 tutorial

Add a tutorial introducing working with JSON in Go, using
encoding/json/v2.

This tutorial is a direct descendant of the https://go.dev/blog/json,
which was the previous canonical introduction to working with JSON.

By placing this a tutorial outside of the blog, it can be a living
document, updated over time as things change. On the other hand, a blog
post is better as a snapshot in time. The old blog post is left in
place, but with a note pointing to the newer tutorial.

Most of the changes from the original blog post are pretty minor.

Links point to encoding/json/v2 instead of encoding/json.

References to `interface{}` are updated to `any` through.

The reference to net/rpc/jsonrpc as an example of JSON code is removed,
as that package doesn't use v2. I don't have a good alternative to link
to.

The biggest change is the json.Encoder/json.Decoder section. This
section previously served three purposes: (1) show how to
marshal/unmarshal with an io.Reader/io.Writer, (2) show how to unmarshal
a sequence of distinct JSON objects, and (3) show to mutate a JSON
object.

(1) is the most important and I've replaced with a short section
describing MarshalWrite/UnmarshalRead.

(2) and (3) are, IMO, much more niche. I could add some jsontext
examples, but I'm not convinced these are important enough to include in
an introductory blog post.

I've replaced those with an overview of custom marshalers, as I think
it's important to introduce users to how to provide custom marshaling
when the defaults are not sufficient.

For golang/go#71497.

Change-Id: Ia24de4c6c8f5c5c3501d7cb35968fe2e6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/website/+/813440
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>