Convert render_static_html.rb to rspec

What does this MR do and why?

Related: Tracking Issue: Implement GLFM scripts per the ... (#361241 - closed)

Converts render_static_html.rb to rspec

• In order to fully exercise the backend markdown rendering stack, we need to call via the API instead of directly calling Banzai.
• We also want easier and more complete control over fixture data, so we want to use the transactional fixture test environment instead of dev environment
• For both of these reasons, we are converting the render_static_html.rb script to be based on rspec instead of a plain ruby executable script.
• This MR also adds some basic fixtures (e.g. project, snippets, wiki) which will be needed to support future example snapshot entries.

See also docs on update-example-snapshots.rb script: https://docs.gitlab.com/ee/development/gitlab_flavored_markdown/specification_guide/#update-example-snapshotsrb-script

See the GLFM Specification Guide development documentation for much more context on all of this work: https://docs.gitlab.com/ee/development/gitlab_flavored_markdown/specification_guide

How to set up and validate locally

1. Run scripts/glfm/update-example-snapshots.rb and ensure there are no failures or changes (expected changes to *.yml files may occur after a rebase).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.