docs: update rclone about docs

Create a full loop of documentation for rclone about, backends overview
and individual backend pages.

Discussion:
https://github.com/rclone/rclone/pull/4774 relates

Previously pull was requested, in part, under ref
https://github.com/rclone/rclone/pull/4801

Notes:
Introduce a tentative draft see-link format the end of sections to try
rather than lots of in-para links.

Update about.go incl link to list of backends not supporting about.

In list of backends not supporting about, include link to about command
reference.

I appreciate there may be decisions to make going forward about whether
command links should be code formatted, and using proper pretty url
links, but I have fudged that for now.

Update backend pages that do not support about with wording used
previously for ftp - it is in passive voice but I can live with it. (my
own wording and fault). The note is applied to a limitations section. If
one does not already exist it is created (even if there are other
limitations with their own sections)

docs/content/http.md

@@ -186,3 +186,13 @@ If you set this option, rclone will not do the HEAD request.  This will mean
 - Default:     false
 
 {{< rem autogenerated options stop >}}
+### Limitations
+
+`rclone about` is not supported by the HTTP backend. Backends without
+this capability cannot determine free space for an rclone mount or
+use policy `mfs` (most free space) as a member of an rclone union
+remote.
+
+See [List of backends that do not support rclone about](https://rclone.org/overview/#optional-features)
+See [rclone about](https://rclone.org/commands/rclone_about/)
+