Create docs for installing and using Code References

[asaphko]

We currently don't have any customer-facing documentation for this feature.

We should add a page in our docs that explains about the feature, covers the use cases and benefits, and includes full instructions for installation of the feature both in our application and connecting it to a repo(s).

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• Reminder to update https://github.com/Flagsmith/ci/blob/main/README.md with a link to documentation.

[matthewelwell]

Dropping the points estimate (from 3 to 2) on this task with the expectation that we are delegating this task to Writechoice.