[Docs]Add documentation for "Backup only new photos" feature

[Shanthy17]

Description

This PR adds comprehensive documentation for the "Backup only new photos" feature in Ente Photos.

Changes:

1. Feature guide (docs/docs/photos/features/backup-and-sync/index.md):

   • Added new section explaining the "Backup only new photos" feature
   • Documented how to enable the setting on mobile
   • Explained the folder selection prompt behavior
   • Provided instructions for disabling the feature
   • Listed common use cases (new devices, privacy, storage management)
   • Added FAQ link to the Related FAQs section

2. FAQ (docs/docs/photos/faq/backup-and-sync.md):

   • Added FAQ entry answering "Can I backup only new photos without uploading my existing library?"
   • Explained how the cutoff timestamp works
   • Linked to the detailed feature guide

Tests

N/A - Documentation only changes

https://claude.ai/code/session_01Y6BbEMFS6vZq2p32igWbWu

[setalp]

a) THis can be done at the time of signup/login as well
b) What we do is backup from that day minus 7 days. So there is something for the user to see and then use the app instead of it being empty

[Shanthy17]

added the change