Use more Redoc render features (REST API)

[pedro-psb]

Problem

The Redoc API render engine has some under-used features (attached below).

Approach

Actually, the changes that can make it better may not belong to pulp-docs itself, but I'll put here to expose the big picture.
We should investigate how we can change DRF/spectacular/??? to make use of those.

To test the API website that originated the snapshots, run the code below.
Notice the relevant schema reference is this.

podman pull redocly/redoc  # from docker registry
podman run --rm \
    -p 8080:80 \
    -e SPEC_URL=https://raw.githubusercontent.com/Redocly/redoc/refs/heads/main/demo/openapi.yaml \
    redocly/redoc

Snapshots

Section groups

We can use sections like "Content Management", "Auth", "Admin", etc.
An opportunity to logically group pulp entities from the user perspective.

Text Sections

We can write text section that appear in the navigation.
This can be used for presenting useful links and info.

Some ideas:

Listing links to supported API pages (and provide the Redoc for those, of course):

## Supported versions

- [3.39](site:pulpcore/restapi/3.39/)
- [3.22](site:pulpcore/restapi/3.22/)
- [3.21](site:pulpcore/restapi/3.21/)

Shortcut links to other plugin API docs:

## Other plugins

[pulp_file](...) - [pulp_python](...) - [pulp_rpm](...) - etc

Deprecated field and endpoint

Just looks better and more obvious that you shouldn't use it.