docs(CONTRIBUTING): refactor, add versioning, remove nightlies

[thehowl]

This PR modifies our CONTRIBUTING guide, adding a section on an informal versioning scheme we are temporarily adopting to communicate breaking changes on the Gno repository.

It also removes CI workflows for nightly builds; while retaining the master builds.

After we merge this, I'll proceed to release the 0.1.0 :)

There is a bit of a re-organization of CONTRIBUTING I batched together with these changes, to update some outdated information, link to the documentation where necessary, and provide information on new policies and suggestions adopted in the repository; including disencouraging rebases and giving write access to maintainers.

Contributors' checklist...

• [x] Added new tests, or not needed, or not feasible
• [x] Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• [x] Updated the official documentation or not needed
• [x] No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• [x] Added references to related issues and PRs
• [x] Provided any useful hints for running manual tests
• [x] Added new benchmarks to generated graphs, if any. More info here.