Transition PSA and API docs to Asciidoc, CI artifacts

[rcgoodfellow]

This PR does a few things, building on the work of @jafingerhut and @Dscano

Moving remaining docs to Asciidoc

• PSA specs and charter
• API charter

CI updates

The asciidoc-build.yml Workflow has been updated to separate the CI process into two phases. The first phase runs on any branch of any fork. It builds the P4, PSA, and Charter docs and produces them as Github Workflow artifacts. This way pull requestors and reviewers can always see the results of CI. The artifacts produced can be seen in the CI output of this PR. Each artifact is a self-contained archive containing HTML and PDF document builds. The artifacts also contain image folders along with HTML documents so they can be fully rendered from the file system once downloaded.

The second phase downloads the artifacts produced by the first phase, and uses them to populate the gh-pages branch of the main repo. This phase only runs on the main branch of the official p4lang/p4-spec repository.

This split is an attempt to address

• CI: Create a CI job that builds the spec PDF and HTML for a PR p4lang/p4-spec#1301

[jafingerhut]

Thanks for those changes! I want to try it out locally on my system to see if I might be able to restore the use of the "include" mechanism that both Madoko and AsciiDoc have, for the places where it was used in the PSA specification, and to look at the generated PDF for the PSA spec quickly to see how it looks.