Add custom user home page to landing page docs

[tomnof]

Summary

• Updates the Custom Organization Landing Page docs to include the landing page priority logic and link to personal home page settings
• Adds a new "Home page" section under Personal Settings in Account Management docs with instructions for setting a personal home page
• Cross-links between both pages for discoverability

Test plan

• Verify the custom landing page renders correctly on preview
• Verify the account management page renders correctly on preview
• Confirm cross-links between pages resolve properly

🤖 Generated with Claude Code

[tomnof]

⚠️ Do not merge until feature is released.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/tom.nof/add-custom-user-landing-page/account_management/
• https://docs-staging.datadoghq.com/tom.nof/add-custom-user-landing-page/account_management/org_settings/custom_landing

[maycmlee]

Looks great, just a tiny nit