docs(cloud): add Import/Export RDB documentation

[Copilot]

Adds documentation for the Import/Export RDB feature under Cloud DBaaS → Features.

New page: cloud/import-export-rdb.md

• Accessing the feature: login → select running instance → Import/Export RDB tab
• Manual Export: Temporary URL (1 hr), GCS, S3
• Manual Import: file upload (≤2 GB), URL (including the temporary export URL), GCS, S3, another instance
  • Instance is flushed before import; automatically rolled back to pre-import state on failure
• Scheduled Import/Export (Pro & Enterprise only):
  • Export → GCS, S3
  • Import → GCS, S3, another instance (URL not supported for scheduled imports)
  • One active scheduled import job per instance
  • Period: hours (min 1); start offset: 0, 15, 30, or 45 minutes

Supporting changes

• cloud/features.md: added has_children: true + Import/Export RDB summary section with "Learn More" link
• .wordlist.txt: added GCS to pass pyspelling CI

[dudizimber]

@copilot we also allow to import RDBs from a URL (could even be the temporary export url). This method is not supported in the scheduled imports though

[Copilot]

@copilot we also allow to import RDBs from a URL (could even be the temporary export url). This method is not supported in the scheduled imports though

Added in the Import section as a URL method, with a note that it is not available for scheduled imports. The description also mentions that the temporary export URL can be used as a source. Commit: docs(cloud): add URL as a manual import source for Import/Export RDB