Update VSA development docs

What does this MR do and why?

This MR updates the VSA development docs.

• Add feature availability table
• Remove outdated section
• Add local GDK instructions
• Link to the Aggregated backend doc

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #337065 (closed)