Add v3 docs (and convenience scripts for working with docs)

[loreanvictor]

Checklist

• I documented the TypeScript code using JSDoc style (NA).
• I added multiple screenshots/screencasts of my UI changes
• I translated all the newly inserted strings into German and English (NA).

Motivation and Context

The documentation is outdated and needs updating. Specifically, the new V3 schema, and the new helper functions for working with Apollon UML models in a safe and backwards compatible manner, should be documented.

Description

This PR adds the following:

• Documentation for new Apollon v3 schemas and helper functions.
• New scripts in package.json for building docs and previewing them locally with auto-rebuild on change.
• A fixes for a minro dependency issue in docs: the docs require Jinja < 3.1, but a higher version was being installed without explicitly specifying the version constraint (see this issue).
• Instructions on README about how to build and work with the docs.

Steps for Testing

1. npm run prepare
2. npm run docs:prepare
3. npm run docs:build to build the docs in docs/build/html
   -- or --
   npm run docs:watch, to preview the docs on localhost:8088

The following requirements should be met:

• The scripts should work and should be explained properly by the README
• Apollon API entry on the docs (built or previewed) should be updated to reflect model 3 schema
• A new UML Model Helpers entry on the docs (built or previewed) should be found, detailing the helper functions with sufficient clarity.

Screenshots