changelog syntax fiddling tooling

We have two programs for trying to make changelogs match the finicky style that the style guide requires.

• maint/changelog-syntax-fiddle:

  • 42 loc, including head comment
  • Doesn't always add full stops
  • Needs manual selection of the most recent release section

• format_md_links:

  • 265 loc
  • Corrupts the file by deleting headings or sometimes combining them with links (see eg the output if run on 9bd7ecbb)
  • Doesn't seem to always put the links in parens. Again, see above.
  • Needs telling the filename but is otherwise automatic.

Both of these are mentioned in Release.md, in two separate places (!)

IMO we should have one tool and it should work properly. The isntructions to use it should be in one place in the release instructions.