Complete Documentation folder restructuring

[lucperkins]

This PR completes the Documentation folder restructuring begun in #10596. It removes the docs folder from the root and adds some _index.md files necessary for constructing the site's table of contents in Hugo.

[codecov-io]

Codecov Report

Merging #10615 into master will decrease coverage by 0.28%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10615      +/-   ##
==========================================
- Coverage    71.8%   71.51%   -0.29%     
==========================================
  Files         393      393              
  Lines       36628    36628              
==========================================
- Hits        26302    26196     -106     
- Misses       8502     8582      +80     
- Partials     1824     1850      +26

Impacted Files	Coverage Δ
proxy/grpcproxy/register.go	80.55% <0%> (-13.89%)	⬇️
pkg/fileutil/read_dir.go	91.3% <0%> (-8.7%)	⬇️
etcdserver/api/v3rpc/lease.go	69.31% <0%> (-7.96%)	⬇️
pkg/tlsutil/tlsutil.go	86.2% <0%> (-6.9%)	⬇️
pkg/fileutil/purge.go	65.9% <0%> (-6.82%)	⬇️
pkg/adt/interval_tree.go	87.08% <0%> (-4.81%)	⬇️
pkg/logutil/zap_grpc.go	47.61% <0%> (-4.77%)	⬇️
pkg/transport/listener.go	54.5% <0%> (-3.8%)	⬇️
pkg/testutil/recorder.go	77.77% <0%> (-3.71%)	⬇️
etcdserver/api/v3election/election.go	66.66% <0%> (-2.78%)	⬇️
... and 16 more

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[lucperkins]

Could somebody possibly advise me on how I can disable the CI Markdown checks? It's not clear to me that those checks are compatible with the new documentation system.

[lucperkins]

I'm going to close this and submit a new PR, which I suspect will be dramatically simpler than rebasing here