Render documentation

[dmke]

This moves most of the contents from the README into /docs.

I want to render the content in two places:

1. https://github.com/digineo/texd/blob/docs/docs/README.md should show the latest documentation. After merging, the URL will be .../blob/master/..., and tagged releases can have their stable docs at .../blob/vX.Y.Z/...
2. http://localhost:2201/docs should render the documentation for the instance running on localhost:2201.

Several files exist to support instance-local rendering. All of them will be embedded in the binary:

• docs/docs.yml defines the navigation tree
• docs/docs.go embeds all Markdown and support files, and provides an HTTP handler to render it
• docs/docs.html contains the layout, similar to service/ui.html

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.76%. Comparing base (d9aa4f2) to head (1a7436b).
⚠️ Report is 216 commits behind head on master.

Files with missing lines	Patch %	Lines
service/service.go	40.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage   80.90%   80.76%   -0.15%     
==========================================
  Files          30       30              
  Lines        1592     1596       +4     
==========================================
+ Hits         1288     1289       +1     
- Misses        243      244       +1     
- Partials       61       63       +2     

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[dmke]

Superseded by #211