Update WebGL contributor docs

[davepagurek]

This brings some initial changes from our STF documentation work into the repo.

It includes two Google Docs converted to markdown via https://davepagurek.github.io/google-docs-to-markdown/. After passing it through the converter, I had to do a bit extra manual work:

• I converted Markdown images to the <image> tag to manually specify a width to make the SVGs more readable
• I updated image URLs to be relative paths to the assets folder
• I manually added js as the language to the code fence blocks

Here's a link to one file directly, so you can see how it looks when reading it on Github: https://github.com/davepagurek/p5.js/blob/0d19d92ee46e5ae2a3568354106e8fc0b6101fc9/contributor_docs/webgl_mode_architecture.md

PR Checklist

• npm run lint passes
• [Inline documentation] is included / updated
• [Unit tests] are included / updated

[davepagurek]

@limzykenneth This doc links to the other doc. Right now I have this linking to an md file with a relative path, but I'm not sure if that will get converted when put onto p5js.org. Should we just do this and add link conversion to the requirements when parsing md(x) files for the site?

[limzykenneth]

Yeah let's leave it as is for now if the link is currently working. We'll review if we need to change it when the website has more structure.