[WIP][docs] Docs sidebar/navbar restructure

[nickfrosty]

Problem

The Solana documentation sidebar and primary navbar are difficult/cumbersome for navigating around the core docs site.

Summary of Changes

• separated the sidebar's for each of the major sections into separate, more digestible chunks
• added footer links for each of the major docs sections
• updated docs home page card "Learn how Solana works" to link to the introduction page
• updated a few page titles to be more specific (for future SEO)
• updated the docs home page card "Start Building" to be more specific
• updated the accepted and implemented proposal sidebars to auto generate based on the documents in their respective folders
• updated the "accepted" and "implemented" proposals section landing pages to add clarity
• created a proposals doc landing page to explain more about proposals (including a guide to submit new proposals)
• updated the CONTRIBUTING file to link to the new guide on submitting proposals
• assorted sidebar links relocated to different sidebar sections (listed below)

Sidebar Links Relocated to Other Sidebar Sections:

• geyser-plugin now under the validator sidebar
• wallet-guide now under the introduction sidebar
• economics_overview now under the introduction sidebar
• command line wallets section now under the command line sidebar

Fixes #26699