Infra: Faster incremental builds, up to 5.4x

[hugovk]

Fixes #3477.

Two changes:

• Only rebuild index files when metadata changes. This was slow, likely because they had to read in and process all the PEP files.
• Split the search index build to its own make target, run on CI/RTD for deploys/previews. We don't need this for most local builds, it adds some 3-5s, and you can specify it if you need it.

PEPs built	main	PR	Speed up
all	42s (4.6s)	30s	1.4x
1	27s (3.5s)	5s	5.4x
0	25s (3.5s)	5s	5.0x

Where:

• all: make clean html
• 1: touch peps/pep-0008.rst; make html
• 0: make html
• Time in brackets is time for search rebuild.

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📚 Documentation preview 📚: https://pep-previews--4848.org.readthedocs.build/