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- Delete out-of-date comments in `XMLreference.rst`.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

doc/changelog.rst

@@ -13,7 +13,7 @@ API changes
 #. Added ``mj_versionString``, which returns human-readable string that represents the version of the MuJoCo binary.
 #. Converted leading underscores to trailing underscores in private instances of API struct definitions, to conform to
    reserved identifier directive, see
-   `C standard: Section 7.1.3 <www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf>`__.
+   `C standard: Section 7.1.3 <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf>`__.
 
    .. attention::
       This is a minor breaking change. Code which references private instances will break. To fix, replace leading