Add documentation for cBioPortal security measures.

[zainasir]

Describe changes proposed in this pull request:

• Add a new page to the cBioPortal documentation site detailing the security measures employed in our development and release workflows.

Checks

• The commit log is comprehensible. It follows 7 rules of great commit messages. We can fix this during merge by using a squash+merge if necessary
• 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.
• 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!
• Make sure your PR has one of the labels defined in https://github.com/cBioPortal/cbioportal/blob/master/.github/release-drafter.yml

Any screenshots or GIFs?

If this is a new visual feature please add a before/after screenshot or gif
here with e.g. Giphy CAPTURE or Peek

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[inodb]

i don't think we use these section demarcations in other markdown files but prolly fine (not sure exactly how this looks in retype)

[zainasir]

Yes, we don't use it anywhere. But it does look sick!

[inodb]

This looks great! Thanks so much for putting this together!