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-platforms.md

@@ -1,8 +1,8 @@
-Platforms
-=========
-
-We maintain the list of supported platforms on our wiki now, and how to
-build and install SDL for those platforms:
-
-    https://wiki.libsdl.org/Installation
-
+Platforms
+=========
+
+We maintain the list of supported platforms on our wiki now, and how to
+build and install SDL for those platforms:
+
+    https://wiki.libsdl.org/Installation
+