[Enhance] replace markdownlint with mdformat for avoiding installing ruby

[ly015]

Motivation

Ref: open-mmlab/mmcv#1936

Modification

BC-breaking (Optional)

Use cases (Optional)

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit tests to ensure correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #1382 (722d348) into master (f8b79fe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1382   +/-   ##
=======================================
  Coverage   83.15%   83.15%           
=======================================
  Files         225      225           
  Lines       18997    18997           
  Branches     3392     3392           
=======================================
  Hits        15797    15797           
  Misses       2364     2364           
  Partials      836      836           

Flag	Coverage Δ
unittests	83.09% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9bc3658...722d348. Read the comment docs.