Store pdf documentation documents in markdown

[Slides](https://github.com/google/highway/blob/master/g3doc/highway_intro.pdf) and [instruction template](https://github.com/google/highway/blob/master/g3doc/instruction_matrix.pdf) are only stored as pdf documents, for easier maintenance and updates it would be helpful to have them stored as text files in a markup language.

For the slides one can use tools such as:
- https://github.com/Om3rr/markdown-to-googleslides
- https://github.com/adamzap/landslide
- https://remarkjs.com
- https://pandoc.org/

to convert to other formats. HTML slides would be nice to have at https://google.github.io/highway/en/master/presentation/index.html

One could possibly also use the webassembly version of highway to allow one to run code in the browser though may need to test how well [emcc](https://emscripten.org/docs/tools_reference/emcc.html) can build highway.

The instruction matrix can be stored as a markdown table and converted to pdf and/or html with highlighting as markdown tables do not display well as plain text without careful formatting.

Start with g3doc/highway_intro.pdf and g3doc/instruction_matrix.pdf, then review the linked presentation target and the listed conversion tools. Define markdown sources that preserve the slides and instruction matrix, with a reproducible path to the requested PDF and/or HTML outputs; done means both documents are maintainable as text and render acceptably in the documentation site.

Written by the indexing model from the issue text.