Update template and checks

[jmesnil]

• Update the template to move the tracking issue and the stability level to the front matter
• When a PR is open, check that these fields are valid
• Generate a HTML page for the proposal with these fields.

This PR updates the template to be more descriptive about its expected content.

As a demo, I opened a "dummy" proposal in my fork that was missing the issue and stability-level in the front matter:

• https://github.com/jmesnil/wildfly-proposals/pull/14
  • The missing-issue and invalid-stability-level labels were automatically added to the PR
• I fixed my PR
  • The labels were removed as the checks passed
  • the asciidoc displays the front matter: https://github.com/jmesnil/wildfly-proposals/blob/main/bootable-jar/dummy_featyre.adoc
• The PR was merged and the metadata were added to the generated proposal page:
  • https://jmesnil.github.io/wildfly-proposals/bootable-jar/dummy_featyre.html
• If the stability-level field is valid in the front matter, add a label to the GitHub PR of the form stability-level/XXX (as seen on my fork at https://github.com/jmesnil/wildfly-proposals/pull/15)

Before that PR is merged, the following labels must be created:

• missing-issue - The analysis document is missing an issue tracker in its metadata
• invalid-stability-level - The analysis document is missing a valid stability level in its metadata
• stability-level/experimental - Feature is at the "Experimental" stability level
• stability-level/preview - Feature is at the "Preview" stability level
• stability-level/community - Feature is at the "Community" stability level
• stability-level/default - Feature is at the "Default" stability level