docs: add IMAP Email server reference page

[kevinhcolgan]

Summary

• Adds a new reference page for the IMAP Email MCP server (nexus/reference/servers/imap-email.mdx)
• Adds the page to the sidebar navigation in docs.json (alphabetical order, between Hunter.io and Linear)

Details

The IMAP Email server provides read and management access to any IMAP-compatible email provider (Gmail, Outlook, Yahoo, self-hosted). Content is based on the Confluence spec.

The page covers:

• Overview and setup instructions (IMAP host, username, password, port)
• App-specific password guidance for 2FA providers
• All 12 available tools organized by category (folders, reading, search, attachments, management)
• Use cases with example prompts
• Cross-links to Gmail and MS365 Mail servers for sending capability

Test plan

• Verify the page renders correctly on the Mintlify preview
• Confirm navigation link appears in the MCP Servers sidebar
• Check that cross-links to Gmail and MS365 Mail pages resolve correctly