document pip's deprecation, removal and behavior change policies

[qwcode]

over the last few releases we've evolved into having an informal policy for deprecations, removals and behavior changes. let's document it in our development section.

[qwcode]

and when saying something is "Deprecated" in the changelog, we should be clearer on the expected date of removal, to prevent conversations like this paylogic/pip-accel#22

[pradyunsg]

@pypa/pip-committers Would putting a section discussing this on the "Development" page make sense for this?

[chrahunt]

Seems to be fixed per the latest stable docs here (from #5526), so I will close this.

[pradyunsg]

I'll note that we'd actually first documented this in #5328.