[Docs] Use docs link of other projects and add MMCV docs.

[mzr1996]

Motivation

Use docs link of other projects and add MMCV docs.

Modification

• docs/conf.py
• docs_zh-CN/conf.py
  And fixed the misused documents, it should be documentation

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #511 (590587f) into master (72cffac) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #511   +/-   ##
=======================================
  Coverage   79.42%   79.42%           
=======================================
  Files         106      106           
  Lines        5959     5959           
  Branches      962      962           
=======================================
  Hits         4733     4733           
  Misses       1095     1095           
  Partials      131      131           

Flag	Coverage Δ
unittests	79.42% <ø> (ø)

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[Ezra-Yu]

LGTM.