Add contributing section to the website

From discussion in slack:

- List `beginner-friendly` and `help wanted` tagged issues (ex: https://hapijs.com/contribute), we might need to do a round of issue tagging/cleanup first.

- At the minimum link to [CONTRIBUTING.md](https://github.com/babel/babel/blob/master/CONTRIBUTING.md), but we probably want to integrate it?

- Link to resources like [babel-handbook](https://github.com/thejameskyle/babel-handbook/), [AST explorer](https://astexplorer.net/), etc.

Review CONTRIBUTING.md, the Babel handbook, and AST Explorer links cited in the issue, then inspect the website's existing contribution or navigation pages. Add a contributing section that links the requested resources and explains how to find beginner-friendly and help-wanted issues; the section should be reachable from the website and reflect any agreed issue-tagging cleanup.

Written by the indexing model from the issue text.