docs: fill stub pages (FAQ redirect, security section intro, roadmap redirect)

[merlynzawadi]

This pull request makes the following changes:

• Fills the empty docs/faq.md stub with a redirect to the full
  frequently-asked-questions.md page and answers the existing
  unanswered "How can I get involved?" question
• Fills the empty docs/untitled/README.md stub with an introduction
  to the privacy & security section, mapping each page to its intended
  audience (users, deployment admins, deployment hosts)
• Fills the empty docs/getting-started/development-process/roadmap.md
  stub with a redirect to the full roadmap page and quick links to
  relevant GitHub issue filters

Note: the docs/untitled/ folder name appears to be a GitBook artifact.
Consider renaming it to security or privacy-and-security in a
follow-up PR to improve discoverability — not included here to avoid
breaking existing links.

Test checklist:

• Verified all internal doc links resolve to existing files
• Confirmed {% page-ref %} syntax matches existing usage in the repo
• I certify that I ran my checklist

Fixes ushahidi/platform# .
Ping @ushahidi/platform

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