*: fix broken links in markdown

[gyuho]

Replace #7864.
Fix #7861.

Only cherry-pick the changes for backporting.
Will integrate with marker once it can handle the [...] in markdown headers.

[xiang90]

lgtm

[fanminshi]

lgtm

[heyitsanthony]

[Branch](Branch)?

[codecov-io]

Codecov Report

Merging #7871 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #7871      +/-   ##
=========================================
+ Coverage   75.69%   75.7%   +0.01%     
=========================================
  Files         332     332              
  Lines       26203   26203              
=========================================
+ Hits        19835   19838       +3     
+ Misses       4935    4929       -6     
- Partials     1433    1436       +3

Impacted Files	Coverage Δ
clientv3/namespace/lease.go	43.75% <0%> (-31.25%)	⬇️
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-13.89%)	⬇️
proxy/grpcproxy/lease.go	79.04% <0%> (-9.53%)	⬇️
auth/simple_token.go	86.79% <0%> (-6.61%)	⬇️
proxy/grpcproxy/leader.go	93.87% <0%> (-6.13%)	⬇️
pkg/testutil/recorder.go	66.66% <0%> (-3.71%)	⬇️
etcdserver/api/v3rpc/lease.go	96.42% <0%> (-3.58%)	⬇️
etcdserver/api/v3election/election.go	64.7% <0%> (-2.95%)	⬇️
etcdserver/api/v3rpc/util.go	52.77% <0%> (-2.78%)	⬇️
... and 14 more

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