[v15] Edit docs CI workflows to accommodate Docusaurus

[ptgott]

Backports #48388

• Edit docs CI workflows to accommodate Docusaurus

• Copy the gravitational/teleport source into the content directory of a gravitational/docs-website clone and overwrite the git-update yarn script with a no-op command. With way, we can use the gravitational/teleport clone across multiple workflow steps (e.g., the prose linting step), and can identify a branch to pull using the actions/checkout defaults. Name the submodule directory current to match the expectations of the Docusaurus site.

• Remove the Vercel preview workflow. After the docs engine migration, we'll no longer need to deploy a preview to Vercel in order to show docs authors what their potential changes look like on a rendered docs site.

Also fixes spelling errors caught by a later version of cspell.

• Fix shellcheck issues in doc-tests.yaml script

• Add exit commands for failed cds
• Don't read from and write to package.json in the same pipeline
• Add double quotes around variables

• Use latest Docusaurus site config schema

• Fix the Prepare docs site configuration step

• Remove unneeded cds
• Use the correct version name in the config.json prepared for the Docusaurus site