Add support for nesting GLFM examples in H3s

What does this MR do and why?

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

Allows GitLab Flavored Markdown examples in the glfm_canonical_examples.txt file to be nested within H3 headings.

Previously they could only be nested with H2 headings.

Also adds a corresponding third-level to the automatic hierarchical numbering of examples in the GLFM Example Snapshot files.

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

Tasks

• Add logic supporting H3s
• Update corresponding docs
• Update all existing example snapshot files to new format

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).
2. Review documentation changes in the MR for correctness.

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.