gh-119786: cleanup internal docs and fix internal links

[picnixz]

Just running a big "Format all" on the .md files. There are some links that were wrong.

There are two commits:

• one commit for cleaning up everything automatically
• one commit that restores the double spaces after a period. We could have unified everything but I wanted to keep the diff as small as possible. If needed, I can drop that commit.

• Issue: create an internals documentation folder in the cpython repo #119786