Add colour to PR Template and tweak CONTRIBUTING and README files

[lildude]

Description

This PR:

• Adds a colour and explanation to the "I am adding a new language" section of the PR template so we can start documenting why a colour has been chosen. This will help in future when there is a request for a colour change.
• Updates the contributing guidelines to make it clear we don't wan't "hello world" samples and advises on adding two samples if the new extension matches another.
• Rewords a few things, removes dead references and updates the list of active maintainers.
• Adjusts the formatting of the CONTRIBUTING.md and README.md files to make them easier to read in plaintext and to highlight certain callouts.

Checklist:

N/A

[Alhadis]

Adjusts the formatting of the CONTRIBUTING.md and README.md files to make them easier to read in plaintext

Could we increment the list-items instead of numbering everything 1....? I know this makes diffs a bit noisier when items are added or removed, but such edits don't happen very often, and having properly-numbered items helps establish a sense of location for the reader.