docs: add contributing docs

Description

This merge request adds CONTRIBUTING.md (both to the repo and linked to the online docs) to help those who want to contribute. Also, adds issue and mr templates that need to be configured after merging using the instructions at https://docs.gitlab.com/ee/user/project/description_templates.html

Supporting information

Grove template's contributing guidelines are pointing to this guideline to ensure we need to maintain it only at one place.

Testing instructions

Steps to test the changes:

1. Checkout the changes
2. Setup a virtualenv with mkdocs installed
3. Run mkdocs serve
4. Proofread the changes

Dependencies

• grove-template!19 (merged)

Screenshots

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers are including the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION is updated
• The grove-template repository is updated

Additional context

This MR was opened using the template's structure.