CTRT: Improve object storage documentation

Problem to solve

The object storage docs contain a lot of information. This issue is about applying the CTRT styleguide.

For technical-writing#749

Proposal

• Give descriptive titles to all headings.
• Rename the the Warnings, limitations, and known issues heading to Troubleshooting, and move unrelated sections out of it.
• Move https://docs.gitlab.com/ee/administration/object_storage.html#using-amazon-instance-profiles closer to the S3 example.
• Move common parameters before the main example.
• Merge everything under Azure blob storage, and list all installation methods.