docs: Add README for core/shipped apps

[joshtrichards]

• Resolves: #

Summary

• Adds clear note that not all core or shipped apps are included in nextcloud/server repository
• Adds quick reference table all the shipped but not included here apps (along with links to their respective repositories)
• Adds a quick reference table of all shipped and/or apps
• Adds a quick reference table of the baseline status of all shipped and/or core apps

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

[CarlSchwan]

Did you use a script to generate this README? asking because my fear is that this get out of sync quickly

[nickvergessen]

This and the description is duplicated information doomed to be out of sync.
The first paragraph of this file together with a hint to core/shipped.json is fine and good to add.

I'd skip the rest of this file to avoid inconsistency