Create a porting case study for the BCM2711/Raspberry Pi 4B

[linguini1]

Summary

This PR introduces a porting guide for the BCM2711 and Raspberry Pi 4B based off of the journal entries I made while creating the initial port in #15188.

Impact

A second porting case study is now available for people looking to port a new board or SoC to NuttX. I also created a new directory for storing the porting case studies so that they are listed under a "case studies" heading in the porting guide, making them easy to browse through.

Testing

Built the documentation locally and rendered in the browser to make sure the build passed and the webpages looked correct. Verified that links all work.

[nuttxpr]

[Experimental Bot, please feedback here]

Fill In The Commit Message: This PR contains a Commit with an Empty Commit Message. Please fill in the Commit Message with the PR Summary.

Yes, this PR appears to meet the NuttX requirements. The summary clearly explains the "why", "what", and "how" of the change. The impact section addresses all the relevant points, highlighting the addition of a new feature (the porting guide) and its positive impact on documentation. The testing section, while not showing specific logs, describes the verification process undertaken, which is appropriate for a documentation change. The absence of related issue numbers suggests this might be a new contribution without a pre-existing tracking issue.

[acassis]

Really good Matteo! Congratulations!