Generate extensions document automatically

- [ ] Generate list of extensions from XML
- [ ] Define rules to guarantee uniqueness of anchors and document them in the extension template or extension README
- [ ] Decide what to do with extensions still authored in plaintext and extensions present in the XML but for which there is no specification (yet)

See https://github.com/KhronosGroup/OpenCL-Docs/pull/846 for background and discussions.

Start by reviewing pull request #846 and the repository's XML sources, extension template, and extension README. Resolve the listed questions about generated extension lists, unique anchors, plaintext extensions, and undocumented XML extensions; done means the generation approach and documentation rules are defined for each case.

Written by the indexing model from the issue text.