Add additional sections to homepage getting started guide

To do:

• Write code samples on adding extractors and loaders, managing configuration, entity selection, and contribution
• Write content on adding extractors and loaders: extracted into !1639 (merged)
• Finish content on managing configuration
• Write content on entity selection
• Write content on contribution
• Link from "integration" to new sections
• Fix link anchors, as the headings have changed
• Point existing links around adding custom taps/targets to new section
• Add link/info under "Intrigued?"?

Merge Request Checklist

• Link to an issue
• Include the proposed fix or feature
• [-] Include and update tests for the modified code
• Include a documentation change
• [-] Add a CHANGELOG.md entry in the Unreleased section

Once your merge request is complete, please assign it a Meltano maintainer for review cycle. Once the review cycle finished, the reviewer shall approve the merge request so it can be merged by any Meltano maintainer.

Closes #2016 (closed)