docs: heavy editing to make this happy with latest wikibridge.

The public headers saw lots of cleanups, backporting from SDL3 docs, and
merging with the wiki.

The markdown files in docs/README-*.md were converted to Unix endlines.

docs/README-hg.md

@@ -1,4 +1,4 @@
-We are no longer hosted in Mercurial. Please see README-git.md for details.
-
-Thanks!
-
+We are no longer hosted in Mercurial. Please see README-git.md for details.
+
+Thanks!
+