Pantheon workflows: only deploy to prod/staging when content files are modified

[vfalconisumo]

Purpose of this pull request

This pull request updates the Pantheon workflow files to ignore changes to files that have no impact on site generation (ex: README.md, .clabot, CONTRIBUTING.md).

I did not include build-related files like package.json and docusaurus.config.js because changing the build process itself felt like something that should trigger a deployment workflow. If this is inaccurate, and we don't want builds to run when the build tool is updated, I'm happy to amend.

Minor housekeeping--this PR also removes the on:workflow_dispatch event from the Pantheon workflows. This was left in place from workflow development by mistake.

Select the type of change

• Minor Changes - Typos, formatting, slight revisions
• Update Content - Revisions, updating sections
• New Content - New features, sections, pages, tutorials
• Site and Tools - .clabot, version updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)

Ticket (if applicable)

Asana task