[Doc] Refine installation and fix some docs

[liqikai9]

Motivation

Modification

Refine installation: add description about installing mmdet.

Fix some links in readme.

Fix demo result image due to previous bug.

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

❗ No coverage uploaded for pull request base (dev-1.x@d26acda). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-1.x    #1659   +/-   ##
==========================================
  Coverage           ?   70.34%           
==========================================
  Files              ?      203           
  Lines              ?    11558           
  Branches           ?     2019           
==========================================
  Hits               ?     8130           
  Misses             ?     3086           
  Partials           ?      342           

Flag	Coverage Δ
unittests	70.34% <0.00%> (?)

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