Refactor Makefile to re-use targets via variables

[hugovk]

Reduce duplication by making many targets set a BUILDER variable and call the main html target.

This also makes it more similar to the CPython Docs and PEPs Makefiles, and makes things easier for us to maintain.

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Also remove some redundant echoes. On main, make html ends like:

build succeeded.

The HTML pages are in _build/html.

Build finished. The HTML pages are in _build/html.

We don't need that extra final line. With this PR:

build succeeded.

The HTML pages are in _build/html.

Note: I left some echoes in there for some (unused?) targets like make man that don't finish building.

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📚 Documentation preview 📚: https://cpython-devguide--1207.org.readthedocs.build/

[willingc]

Looks good @hugovk. Much cleaner.