Update Bitbucket troubleshooting section
What does this MR do and why?
As a part of providing assistance in this ticket, Support identified that in some situations a customer may have incorrect proxy configuration values on their Bitbucket server. This can lead to the wrong import URL being used when GitLab attempts the import. The documentation adds a basic section that recommends the user refer to Atlassian's documentation on using reverse proxies.
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.