Break up step in "generate pages for existing tags" tutorial

[tudortimi]

Step 3 on this page could be broken up. The first part could show how to use uniqueTags to return exactly the same array (with tag and all posts). This means that the HTML part doesn't need to change and the user can focus on understanding the JavaScript code a little better. The second part could show how move filtering in to the function, which would require the change to the HTML template.

[tudortimi]

I initially started this as a discussion in the "docs" repo (withastro/docs#10869), but later found this repo.