Add documentation for getting started and writing plugins

[garyttierney]

See https://apollo-rsps.github.io/.

TODO list for documentation to be written. Ideally, we need docs on: building the server, configuring it, extending it, developing plugins for it, and some per-plugin documentation for those with DSLs or public APIs

[garyttierney]

TODOs:

• Documentation to finish writing
  • Listening to server events
  • Exporting code from a plugin
  • Writing plugin tests
• Branding the AsciiDoc output
• Better navigation (search?)
• Generate javadoc and kdoc for the project and plugins

@Major- ?

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (kotlin-experiments@12b4bef). Click here to learn what that means.
The diff coverage is n/a.

@@                  Coverage Diff                  @@
##             kotlin-experiments     #417   +/-   ##
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  Coverage                      ?   23.29%           
  Complexity                    ?      809           
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  Files                         ?      655           
  Lines                         ?    11343           
  Branches                      ?     1591           
=====================================================
  Hits                          ?     2642           
  Misses                        ?     8446           
  Partials                      ?      255

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