[Boost] Write documentation for Boost Cache

This can be tackled after we release an internal beta, while we wait for feedback and prepare for launch.

There is a growing list of "Learn More" links throughout Boost Cache's UI. We need to write a documentation page, or multiple pages that address them all.
- [ ] On the error messages that can appear on the Boost dashboard when setup fails
- [ ] On the "Exceptions" list configuration

Start from the Boost Cache UI's growing list of “Learn More” links and identify the linked setup-failure error messages and the “Exceptions” list configuration. Review the existing Boost Cache documentation structure before choosing one page or multiple pages. Done means each listed link has documentation covering its topic and the pages are ready for launch feedback.

Written by the indexing model from the issue text.