This PR updates the 12-factor reference pages to incorporate the following feedback received during UXR sessions: * Update the headers so that they’re human-readable and indicate the purpose or goal of the section * Clarify that the `parts` snippets are technically optional, but depending on the complexity of the project, could be required. Clarify when the user needs to add the snippet versus when they can safely ignore the snippet * Clarify the parts snippets that include or exclude files. Add this type of snippet overrides the default, and you exclude a file by including the snippet in your project file without the file. Add a reference for more complex ways of including or excluding files (e.g., with glob patterns) * Add more links (start script in the Express framework page, bare base, rocks, etc.) * Add captions to all code snippets so it’s clear that they touch the rockcraft.yaml project file. * Double check and clarify any undefined acronyms (e.g. WSGI in the Flask reference page). I successfully ran `make docs-lint` before opening the PR. I also did not add meta descriptions to avoid conflicts with https://github.com/canonical/rockcraft/pull/1246 --- - [x] I've followed the [contribution guidelines](https://github.com/canonical/rockcraft/blob/main/CONTRIBUTING.md). - [x] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [x] I've successfully run `make lint && make test`. - [x] I've added or updated any relevant documentation. - [ ] In documents I changed, I [added a meta description](https://canonical-starflow.readthedocs-hosted.com/how-to/add-a-page-meta-description/) if one was missing. - [ ] I've updated the relevant release notes.