convert README to markdown

Replace the auto-converted README.md (which carried pandoc title-ref
spans and a garbled Tests heading from a malformed rst underline) with
a clean conversion of README.rst, and remove the rst version.
pyproject.toml already declares README.md as the package readme.
Content is unchanged; bringing it up to date is left for a separate
change.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -10,28 +10,29 @@ submodule like:
     git submodule update
 
 Wikimedia dumps are usually in a compressed format such as 7z (most
-common), gz, or bz2. Wikiq uses your computer\'s compression software to
-read these files. Therefore wikiq depends on [7za]{.title-ref},
-[gzcat]{.title-ref}, and [zcat]{.title-ref}.
+common), gz, or bz2. Wikiq uses your computer's compression software to
+read these files. Therefore wikiq depends on `7za`, `gzcat`, and `zcat`.
 
 # Dependencies
 
 These non-Python dependencies must be installed on your system for wikiq
 and its associated tests to work.
 
--   7zip
--   ffmpeg
+- 7zip
+- ffmpeg
 
-A new diff engine based on [\_wikidiff2]{.title-ref} can be used for
-word-persistence. Wikiq can also output the diffs between each page
-revision. This requires installing Wikidiff 2 on your system. On Debian
-or Ubuntu Linux this can be done via.
+A new diff engine based on [wikidiff2](https://www.mediawiki.org/wiki/Wikidiff2)
+can be used for word-persistence. Wikiq can also output the diffs
+between each page revision. This requires installing Wikidiff 2 on your
+system. On Debian or Ubuntu Linux this can be done via.
 
 `apt-get install php-wikidiff2`
 
 You may have to also run. `sudo phpenmod wikidiff2`.
 
-Tests \-\-\--To run tests:
+# Tests
+
+To run tests:
 
     python -m unittest test.Wikiq_Unit_Test