Documentation improvements for 2.1.1.

- Fix typos and broken links.
- Add documentation for functions introduced in 2.1.1.
- Add copyright header to `FLV.m` and remove `hello.c`.
- Update documentation for `mj_ray` and `mj_contactForce`.
- Delete out-of-date comments in `XMLreference.rst`.
- Add geom:size table to `XMLreference.rst`.

PiperOrigin-RevId: 425318966

.gitignore

@@ -0,0 +1,19 @@
+# Don't commit binaries
+*.a
+*.asm
+*.dll
+*.dylib
+*.exe
+*.lib
+*.out
+*.o
+*.so
+
+# Exclude editor config
+.vscode/
+.vs/
+
+# Exclude temporary folders
+*.egg-info/
+build/
+build_cmake/