#91: documentation updates post November ballot

[brendanreardon]

#91 documentation updates post November ballot.

A summary of changes include:

• CONTRIBUTING.md was updated to fix typos and incorrect references (e.g., referring to Issues within the Discussion header of the document). The example Issue used for Branch and Pull Request naming conventions was previously We have discussions now! #15, which was a Discussion. This was changed to Add examples for Cat-VRS data classes #87 for consistency since it follows the recommended format.
• CONTRIBUTING.md also references the main branch in the Pull Requests section. This was changed to reference the Default branch and hyperlinks to the branches page, since this repository currently uses 1.x as the Default branch and presumably this name will change with subsequent releases.
• Broadened description of Categorical Variant class, from @rhdolin's recommendation.

I also added details to the root README.md that:

• Encourages contributors to run tests locally before pushing commits after contributing to the schema.
• Reminds contributors to commit changes to schema/cat-vrs/def and schema/cat-vrs/json after regenerating def and json files following schema changes.
• Reminds contributors to make and commit changes to docs/source/concepts, if relevant, after contributing to schema and regenerating docs.

A (maybe controversial) change that I added was:

• Changed the maximum pixels of the cat-vrs cat in the root README.md to be 300 pixels, so that users see that there is additional documentation in the README.

Two documentation items that were identified, but can be made into their own Issues are:

• Updating descriptions of concepts within docs/source/concepts/
• CONTRIBUTING.md references a style guide whose link is now broken and the file is currently in docs/tobedeleted/. va-spec still has this file in docs/source.

[DanielPuthawala]

Looks all good to me! And yea, the Readme is functionally improved with a slightly smaller catstronaut. XD