Docs: replace absolute links with relative ones.

[pop]

This change is necessary for some downstream documentation that gets built for https://coreos.com/etcd/docs/latest.

Also absolute links are notoriously not a good idea 😉

CC: @robszumski, @joshix, @crawford

[xiang90]

lgtm

[codecov-io]

Codecov Report

Merging #7667 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7667      +/-   ##
==========================================
- Coverage   73.36%   73.32%   -0.05%     
==========================================
  Files         328      328              
  Lines       26643    26643              
==========================================
- Hits        19547    19536      -11     
- Misses       5625     5635      +10     
- Partials     1471     1472       +1

Impacted Files	Coverage Δ
mvcc/watchable_store.go	68.99% <0%> (-17.45%)	⬇️
pkg/fileutil/purge.go	73.68% <0%> (-7.9%)	⬇️
auth/simple_token.go	86.84% <0%> (-7.9%)	⬇️
etcdserver/api/v3rpc/watch.go	89.52% <0%> (-4.19%)	⬇️
proxy/grpcproxy/watcher.go	89.58% <0%> (-4.17%)	⬇️
proxy/grpcproxy/watch.go	90.97% <0%> (-2.09%)	⬇️
clientv3/watch.go	93.84% <0%> (-1.29%)	⬇️
etcdserver/server.go	79.09% <0%> (ø)	⬆️
rafthttp/msgappv2_codec.go	71.3% <0%> (ø)	⬆️
rafthttp/transport.go	83.05% <0%> (+1.12%)	⬆️
... and 7 more

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