Parent task: Reflect "latest update" and "support level" in docs, examples and code

[stichbury]

Parent task to help users understand before they use docs, examples or code, whether it's recently updated or if they should proceed with caution.

Parent of #2578 #2393 and #2291

[stichbury]

Somewhat/slightly related to #2823 in that this is another label applied to docs but whereas that ticket is all about whether it's new docs or stable, this is old docs/deprecated docs/updated recently.

[astrojuanlu]

Possible implementation:

• https://github.com/mgeier/sphinx-last-updated-by-git
• https://github.com/jdillard/sphinx-gitstamp

[stichbury]

I've created a PR #3547 for the first part of this task and a second issue #3576 to continue it (needs engineering to do this part as it'll be python changes for Sphinx docs build).

[stichbury]

I'm moving this to done but keeping child tasks that aren't done open for now.