add statistical tests in GC in FAQ

[jjgao]

Fix #10198

Describe changes proposed in this pull request:

• added statistical tests in group comparison in FAQ

Checks

• Runs on heroku
• Has tests or has a separate issue that describes the types of test that should be created. If no test is included it should explicitly be mentioned in the PR why there is no test.
• The commit log is comprehensible. It follows 7 rules of great commit messages. For most PRs a single commit should suffice, in some cases multiple topical commits can be useful. During review it is ok to see tiny commits (e.g. Fix reviewer comments), but right before the code gets merged to master or rc branch, any such commits should be squashed since they are useless to the other developers. Definitely avoid merge commits, use rebase instead.
• Is this PR adding logic based on one or more clinical attributes? If yes, please make sure validation for this attribute is also present in the data validation / data loading layers (in backend repo) and documented in File-Formats Clinical data section!

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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[inodb]

we have a ticket to update these, so need to remember to update the FAQ after: #9943

[inodb]

added it to the ticket #9943

[jjgao]

@inodb @dippindots should we merge this and update it later as part of #9943?

[dippindots]

Yes, sounds good. I will merge this one and update it later

[inodb]

LGTM!

[inodb]

added it to the ticket #9943

[tmazor]

Good idea to add this! Looks good to me.