templates: add a PR template for docs content

[ES-Alexander]

Should include:

1. A preview link
   • May need to create a workflow to add the PR number to the link, perhaps replacing a template string (PR_NUMBER?) as an extra step after successful deployment of the preview site?
2. A "documents" section, with links to relevant PRs
3. A "minimally documents" section, with links to relevant PRs, and explanations of what is left to document