Add contribution instructions

[AnneCYH]

Hi @farshidtz ,

The landing page needs to be updated along with this change: https://canonical-real-time-ubuntu-documentation--28.com.readthedocs.build/en/28/#project-and-community

• Link to the CONTRIBUTING.md instead of the repo root.
• Be clearer that this is contributing to the docs only.

  If you wish to contribute to the Real-time Ubuntu docs...

Thanks.

[AnneCYH]

Hey @farshidtz , I see your POV regarding certain points that might seem obvious.

I suggest making some slight revisions below to signpost the docs while accounting for the needs of newer contributors.

Added suggestions to that effect.

Thanks!