chore: Update README

What does this MR do?

As part of gitlab-org/gitlab#370011 we should work on centralizing our development docs into the new Sec Section Development docs.

I'm proposing a new sub-page called something like Sec Section Analyzer Development with the majority of the contents ported from this README (and eventually combined with the random page we have in the handbook, too.

To make the diff review more straightforward, I wanted to first apply the changes to this repo. Once these are finalized I can copy the contents to docs.gitlab.com and isolate the review to grammatical changes.

What are the relevant issue numbers?

gitlab-org/gitlab#370011

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer