[DOCS]: 3dcitydb documentation page

[shrakesh]

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Create 3dcitydb module documentation page which tells about 3d city database and how to enable and disable.

Here are some reminders before you submit the pull request

• [ ] Add tests for the change
• [ ] Document changes
• [ ] Communicate in the mailing list if needed
• [ ] Pass make installcheck
• [ ] Review a PR in return to support the community