Remove phab migration documentation

[silkeh]

Description

As the migration is over, I think all references can be removed to clean up the documentation.

• Remove the Phab migration warnings.
• Remove the Phab migration procedures.
• Remove some leftover Makefile references.
• Rename the .mdx pages to .md to make GitHub render diffs again.

Submitter Checklist

• [x] Squashed commits with git rebase -i (if needed)

[davidjharder]

No notes, looks good to me!