Instructions on how to use Pages templates

What does this MR do?

Adds documentation on how to start a new project using a Pages template easily.

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56991

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards