docs(testing): Add docs for local testing and contributor test workflow

[weiserhase]

This PR adds documentation for running and maintaining the Typst tests suite locally.

What changed

Added a new testing guide with end-to-end local workflow:

• running tests
• prerequisites
• updating snapshots
• creating new tests
• CI parity commands
• common troubleshooting

Updated contribution docs to:

• link to the testing guide
• require running tests before submission

[l0uisgrange]

Hi @weiserhase 👋,

I think we might be better with PR templates than creating a new README.
That way, people will directly have the commands to run when opening their PR.

Also, I wouldn't put the tytanic docs into the .md, since it already exists into the official tytanic documentation.

Could you instead create a template for PRs and put in it the requirements? For example

• naming convention for PRs
• tytanic tests (with a link to tytanic docs)
• require to put screenshot if new component/changed component (provide a table template with "before->after")
• ...

[weiserhase]

@l0uisgrange Yes your right! I will work on this as soon as possible