DB Populator: Allow uploading single manifests, do not require serving JSON files

[pkosiec]

Description

Improve DB Populator:

• Allow uploading (adding & replacing) single manifests
• Do not require serving JSON files, as this is problematic especially when populating Capact available behind Bastion
• Update documentation (including content development guide)

Reason

These changes will dramatically improve experience for Content Developers creating theirs manifests.

Use cases

• As Content Developer, I want to upload manifests into Hub in the easiest and fastest possible way.