Improve inputs docs and add missing `regex` parameter

What does this MR do and why?

Related to Document new input validations (type, options, ... (#429105 - closed)

Improve documentation about the new features related to spec:inputs when including CI configurations.

Simplified the documentation by providing an example with all the possible configurations.

Also added the missing regex input attribute to the keyword reference.

Changelog: other

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.