Add top-level-overview READMEs for whole repo and each plugin

[Parkreiner]

Working on documenting the various issues I'd been collecting ad-hoc.

We need a set of READMEs for each plugin that:

1. Provide a top-level overview for what the purpose of the plugin is
2. Has a "quick start" section for getting someone up and running with the plugin as soon as possible
3. Has a link to more in-depth API docs (which will each be separate READMEs)
4. Has info for how to contribute

On top of that, we'd also want a README for the entire monorepo

@bpmct feel free to edit in more info for this issue if you think we need anything else

[code-asher]

Some other things we might want to do in relation to how the plugins show up in NPM:

• The readme title should probably match the package name?
• Seems NPM uses the first line in the readme as the description, but maybe we want an explicit description field in the package.json.
• Could add keywords in the package.json to make the plugins more discoverable.

[Kira-Pilot]

Closing in favor of https://github.com/coder/backstage.cdr.dev/issues/51 and https://github.com/coder/backstage.cdr.dev/issues/24