docs: add reference check while build docs

[bwanglzu]

Add cross-reference checker for sphinx.

with this PR, several options have been offered to make docstring more robust:

1. make html will show warnings if the internal referenced link is incorrect.
2. make linkcheck same as above, without generating html pages.
3. make coverage check the docstring coverage.

Some basic statistics:

broken links: Found 2724 broken links, ~2550 were broken links in CHANGELOG.md.
coverage: ~40 undocumented methods & classes.
warnings: 2721 warnings, mostly are class or method reference not found.

[github-actions]

Latency summary

Current PR yields:

• 😶 index QPS at 1670, delta to last 3 avg.: -2%
• 😶 query QPS at 32, delta to last 3 avg.: -2%

Breakdown

Version	Index QPS	Query QPS
current	1670	32
0.9.2	1717	32
0.9.1	1684	32
0.8.22	1747	32

Backed by latency-tracking. Further commits will update this comment.

[codecov]

Codecov Report

Merging #1600 (bbcb4f0) into master (692c2d9) will increase coverage by 0.92%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##           master    #1600      +/-   ##
==========================================
+ Coverage   83.92%   84.84%   +0.92%     
==========================================
  Files         128      128              
  Lines        6674     6732      +58     
==========================================
+ Hits         5601     5712     +111     
+ Misses       1073     1020      -53     

Impacted Files	Coverage Δ
jina/peapods/runtimes/jinad/__init__.py	48.38% <44.44%> (ø)
jina/peapods/runtimes/jinad/api.py	54.66% <100.00%> (-0.31%)	⬇️
jina/logging/profile.py	69.84% <0.00%> (-0.56%)	⬇️
jina/drivers/craft.py	100.00% <0.00%> (ø)
jina/types/ndarray/generic.py	100.00% <0.00%> (ø)
jina/drivers/encode.py	94.91% <0.00%> (+0.08%)	⬆️
jina/enums.py	96.59% <0.00%> (+0.09%)	⬆️
jina/jaml/__init__.py	95.93% <0.00%> (+0.09%)	⬆️
jina/flow/base.py	86.58% <0.00%> (+0.10%)	⬆️
jina/clients/helper.py	94.00% <0.00%> (+0.12%)	⬆️
... and 15 more

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