gh-106259: add minimal help target to Makefile

[smontanaro]

Partially adding a help target, but partially just getting familiar @CAM-Gerlach's git new alias.

• Issue: Add trivial "help" target to Makefile.pre.in #106259

[vstinner]

What about putting the documentation there instead? https://docs.python.org/dev/using/configure.html#python-build-system

[smontanaro]

My intent was to put more elaborate target documentation somewhere. Haven't started writing anything yet. Thanks for the suggested destination.

[smontanaro]

What about putting the documentation there instead? https://docs.python.org/dev/using/configure.html#python-build-system

I will look into that as well. Having a "make help" command you can execute when you just need a quick reminder is often helpful. And, it's always right at your fingertips.

[smontanaro]

I made a quick, crude pass at expanding the makefile targets in Doc/using/configure.rst. Some descriptions are essentially unchanged. Other bits I actually expanded on in a hopefully useful way. This still needs more work, but I'm done typing for today, so I'll leave it for now. Feel free to suggest further edits.

[vstinner]

Overall, the change LGTM. I just have some minor remarks.

cc @erlend-aasland @corona10: Do you want to review these changes?

[smontanaro]

Any further comments on this @vstinner or @erlend-aasland, or can we push this to completion?

[erlend-aasland]

I'll let @hugovk, @vstinner and @merwok decide and land, since they've been following up this PR :)

[hugovk]

Looks good! Some small bits and pieces:

[vstinner]

LGTM.

I would prefer to sort targets in make list-targets but it's a minor issue.

[smontanaro]

Thanks. Can someone merge?

[vstinner]

Merged, thanks @smontanaro!