Document PEP517 difference (#4480)
## Summary
PEP 517 build isolation https://github.com/astral-sh/uv/pull/843 has not
yet been mentioned in the PIP compatibility documentation. Add a section
for it.
## Test Plan
Visual inspection only
## Open Questions
> in most cases, swapping out `pip install` for `uv pip install` should
"just work".
In the first non-trivial case I tried, it worked for a short time and
then [started
failing](https://github.com/astral-sh/uv/issues/4069#issuecomment-2186762048).
Is there any data out there on how many top 100 or top 1000 packages
work with PEP 517 build isolation?
How can someone specify `--no-build-isolation` for just one package/line
in `requirements.txt`?
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Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
