Documentation: move design docs to "Documentation", remove "docs"

[gyuho]

Remove "docs"

[gyuho]

@jpbetz @jingyih

[codecov-io]

Codecov Report

Merging #10949 into master will decrease coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #10949     +/-   ##
=========================================
- Coverage   63.65%   63.45%   -0.2%     
=========================================
  Files         400      400             
  Lines       37415    37415             
=========================================
- Hits        23816    23742     -74     
- Misses      11984    12066     +82     
+ Partials     1615     1607      -8

Impacted Files	Coverage Δ
client/client.go	43.13% <0%> (-40.85%)	⬇️
clientv3/namespace/lease.go	43.75% <0%> (-37.5%)	⬇️
clientv3/namespace/util.go	88.23% <0%> (-11.77%)	⬇️
clientv3/leasing/util.go	88.33% <0%> (-10%)	⬇️
clientv3/balancer/balancer.go	86.36% <0%> (-2.28%)	⬇️
proxy/grpcproxy/watch_broadcasts.go	92.53% <0%> (-1.5%)	⬇️
proxy/grpcproxy/watch.go	88.55% <0%> (-1.21%)	⬇️
auth/store.go	45.08% <0%> (-1.15%)	⬇️
raft/raft.go	90.07% <0%> (-0.67%)	⬇️
pkg/proxy/server.go	59.86% <0%> (-0.34%)	⬇️
... and 19 more

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[jingyih]

lgtm