[Docs] Fix TOC of visualization.md and add example images.

[mzr1996]

Motivation

We forgot to add the learning rate schedule visualization to TOC.

Modification

As the title.

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 #513 (35c6b27) into master (b486bb2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #513   +/-   ##
=======================================
  Coverage   79.48%   79.48%           
=======================================
  Files         106      106           
  Lines        5975     5975           
  Branches      968      968           
=======================================
  Hits         4749     4749           
  Misses       1095     1095           
  Partials      131      131           

Flag	Coverage Δ
unittests	79.48% <ø> (ø)

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[Ezra-Yu]

LGTM.