Add SimSiam to the docs

[akihironitta]

What does this PR do?

This PR adds SimSiam to the docs, which was missed in #407.

Let me add "ready-to-go" as this change is quite obvious.

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• [n/a] If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #604 (06dc88f) into master (a675724) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #604   +/-   ##
=======================================
  Coverage   25.20%   25.20%           
=======================================
  Files         118      118           
  Lines        7119     7119           
=======================================
  Hits         1794     1794           
  Misses       5325     5325           

Flag	Coverage Δ
cpu	25.20% <ø> (ø)
pytest	25.20% <ø> (ø)

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