Refactor `include` CI yaml docs

What does this MR do?

Refactor the include CI yaml docs. Notably:

Notably:

• Use include-specific keywords on their own section
• Split examples into their own section
• Add section of using include with extends (example taken from https://gitlab.com/gitlab-org/gitlab-ce/issues/56235)

While at it, I got include and extends one after another, and (minor one) I moved pages down the chain.

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56235.

Author's checklist

• Apply the correct labels and milestone
• 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