Db documentation improvement

[GernotMaier]

Minor improvement to the documentation on how to setup a local DB (I forgot how to do this after my vacation - this is why I checked the docs).

[Copilot]

Pull request overview

Improves the documentation for MongoDB usage in simtools, focusing on clarifying how to browse/update the simulation models DB and how to set up a local containerized copy for development.

Changes:

• Clarifies write-access policy and model-repository-to-database update workflow.
• Reworks local DB setup instructions (adds step-by-step commands and a streamlined upload flow via simtools-db-upload-model-repository).
• Adds a changelog fragment for the documentation update.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
docs/source/components/databases.md	Updates/extends MongoDB documentation, including local DB setup and model upload instructions.
docs/changes/2080.doc.md	Adds a documentation changelog entry for the DB docs improvements.