Rephrase Pages introduction, ensure newbies find their way

This MR updates the user documentation for GitLab Pages.

1. Makes the introduction more accurate by shifting the focus from "free as in free beer" to "what happens technically" and at "no additional cost".
2. Adds a reference to the paragraphs describing how to set up a static site from scratch.

Why was this MR needed?

Working with students aged 12-15 we noticed that the GitLab Pages documentation is not self-explanatory. It needs support from someone experienced (with CI and programming) to get hosting for, e.g., just a simple HTML website work.

Students need to understand why things happen! They want to read up on documentation (sometimes). It's not sufficient (for everyone) to just make things work regardless to the "why".

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary - n/a
• Documentation created/updated
• API support added - n/a
• Tests added for this feature/bug - n/a
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides - n/a
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered - n/a
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan - n/a
• End-to-end tests pass (package-qa manual pipeline job) - n/a

What are the relevant issue numbers?

None