Cleanup PIP package tools and installation documentation.

There are a few users to consider:

1. A developer on TFF wants source at HEAD.
2. A researcher using TFF wants pip package at release in 90% of scenarios and sometimes (ideally rarely) wants nightly, but never wants to build a pip package.
3. A developer making a framework using TFF wants pip release and nightly, but never wants to build a pip package.
4. A researcher upstreaming change to TFF basically has moved from #2 to #3.

Fixes: #878
PiperOrigin-RevId: 321226038