fix(docs): proposals sidebar and landing page

[nickfrosty]

Problem

The Solana docs "proposals" sidebar restructure for #26699

Summary of Changes

• segmented the "proposals" section sidebar from the global sidebar
• updated the accepted and implemented proposal sidebars to auto generate based on the documents in their respective folders
• updated the "accepted" and "implemented" section landing pages to add clarity
• created a proposal doc landing page to explain more about proposals (including a guide to submit new proposals)
• added a footer link to the "proposals" root page
• updated the CONTRIBUTING file to link to the new guide on submitting proposals

Screenshot of the new "proposals" section sidebar:

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[nickfrosty]

Bump to prevent being deleted

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.