Docs: adds guides index file

[ZPain8464]

After the re-org for v20, the 'guides.md' file was removed. This caused the "Guides" navbar button to link to category/guides. This affects links that redirect to docs/guides; even if the redirect file redirects 404ing links to category/guides, it redirects instead to the Pomerium homepage.

If approved I'll create an issue and include it with this PR in the 404 epic for March, as it should redirect docs/guides links to the correct page.

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[backport-actions-token]

The backport to 0-20-0 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-0-20-0 0-20-0
# Navigate to the new working tree
cd .worktrees/backport-0-20-0
# Create a new branch
git switch --create backport-457-to-0-20-0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 6ca95ab5158ab8c7367d84ec1611b034f9caec7a
# Push it to GitHub
git push --set-upstream origin backport-457-to-0-20-0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-0-20-0

Then, create a pull request where the base branch is 0-20-0 and the compare/head branch is backport-457-to-0-20-0.

[ZPain8464]

Fix here #461