docs: add DocC landing page for FerrostarCore

[NAKKA-POOJITHA]

This PR addresses part of #347 by introducing a dedicated DocC documentation landing page for the FerrostarCore framework target.

Changes

• Added FerrostarCore.docc directory structure under apple/Sources/FerrostarCore/.
• Created FerrostarCore.md providing a high-level architectural overview, core concepts, and links to the user guide using proper DocC symbol linking.

[ianthetechie]

Thanks; this is a good start!

From the original issue:

Ideally we'd figure out how to make this mirror the iOS setup docs from the guide. Both use Markdown so this should be scriptable.

I think that we do still need to do this. Though in retrospect maybe my original idea of using a script wasn't the best, since mdbook is more vanilla markdown formatting, and DooC has its own special way of doing links. So, I think we should probably rather try to move the iOS docs out of mdbook and over to here, and adapt the formatting / page structure as needed to Apple/DooC conventions.