Improve documentation for contributing

[dana-gill]

Is your feature request related to a problem? Please describe.
Upon trying to start contributing to librosa to make improvements for the documentation, I encountered a few problems. The first is CONTRIBUTING.md is not accessible via the website, and I had to look through the repository to find it. The second is that the file for Compiling Documentation is not referenced in the Contributing Page. The documentation is unfortunately a bit scattered which makes it difficult for first-time contributors 😞

Describe the solution you'd like
This was talked about in an issue #1478 (comment). To summarize:

• Find a way to auto-include the CONTRIBUTING.md page within the website
• Reference Compiling Documentation in the Contributing Page

Describe alternatives you've considered
For the CONTRIBUTING.md page, it was proposed to link it or convert it into .rst. However, it is required for it to stay as an .md file to fit GitHub's Community Guidelines.

Additional context
Please see #1478 (comment) and #1478 (comment) for further context 😸