Merge branch 'renames' into docs-mw

From Vegard Nossum:

When we rename an .rst file, that also changes the URL for the document
at https://docs.kernel.org/ and results in a 404, which can be anonying
for people who bookmark URLs and/or follow links from search engines
and old changelogs and emails.

In order to be able to fearlessly rename individual documentation files
and reorganize Documentation/, add two scripts:

- tools/docs/gen-renames.py : use git to figure out which .rst files
  have been renamed

- tools/docs/gen-redirects.py : actually generate .html stubs for the
  locations, redirecting to the new locations

The reason for splitting this into two is that trawling git history is
slightly slow (on the order of 20-30 seconds on my laptop) whereas just
generating the HTML files is very fast. This also allows us to cache
the historical renames in Documentation/.renames.txt or add manual
fixups as needed.