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

Merging #35 (1a7436b) into master (d9aa4f2) will decrease coverage by 0.14%.
The diff coverage is 57.14%.

@@            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     

Impacted Files	Coverage Δ
service/service.go	63.46% <40.00%> (-1.19%)	⬇️
service/ui.go	100.00% <100.00%> (ø)

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