docs: initial prompts changes

[TyDunn]

Summary by cubic

Revamps the Prompts and Rules docs to reflect the new prompts workflow and consolidates slash command guidance into the Prompts deep dive. Cleans up Hub block links and removes deprecated content.

• Refactors

  • Prompts: clarify that prompts kick off Agent/Plan/Chat as user messages; Quick Start now uses .continue/prompts and requires isPrompt: true; adds “Agent Prompt Blocks” note with link to block types.
  • Rules: tighten description; keep baseSystemMessage override note; remove deprecated .continuerules section.
  • Hub: update explore links to use query params; dedupe Prompts section; simplify page title.
  • Remove the standalone Slash Commands doc.

• Migration

  • Move prompt files to .continue/prompts and add isPrompt: true in frontmatter.
  • Use agent prompt blocks to expose prompts as slash commands.
  • Update any bookmarks to the new Hub URLs with ?type= filters.