[project-structure] update FileTree example to show content collections

[sarah11918]

Description (required)

Updates the Project Structure page in the Basics section:

<FileTree> updates:

• shows the three markdown posts as a collection, instead of located in /pages/posts/ as a more realistic/helpful example
• adds an example of more helpful pages (about, RSS, and dynamic routing)
• bolds index.astro

src/pages updates:

• moves it to be the first example, as the most important one (do not care that this breaks alpha order)
• Simplifies the (not entirely accurate/comprehensive) wording and emphasizes the link to Pages basic guide instead

Caveats:

• This will may need to be slightly tweaked again for v5 beta when content collections rules change, but this is much more future proof for that branch and isn't technically wrong.

Fun fact:

• First test of submitting a PR through IDX!

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Warnings reference

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.