feat: add blank template

[steveiliop56]

Description

This pull request, adds a simple new page template called blank that allows the end-user to fully customize what's rendered in the ContentPanel without having to worry about CSS hacks to hide things like the PageTitle which are present in the splash template. For more information see #3906

Additionally, it refactors a small section in the Page.astro that handles conditional rendering for pages with a hero image. Hopefully it improves visibility and maintainability in comparison to the previous version.

This pull request does not impact the default Starlight look.

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[trueberryless]

Thanks for this amazing first time contribution 🥳

Overall the changes look good, I would just like to point out some docs changes:

1. Could you add a new section under https://starlight.astro.build/guides/customization/#page-layout which explains the new 'blank' template similar to how you currently explain it in the changeset. One difference to the current changeset explanation tho is that the docs should be more "stateless" and unemotional, meaning that they reflect the current state of how things work, so no need for phrases like "new 'blank' template" or any introduction to reason about the third template.

   I think since this is a new feature, we can add an entire new heading, e.g. something like that (I'm sorry that I can't suggest it in the code directly, since it is outside the diff range):

   ## Page layout
   
   By default, Starlight pages use a layout with a global navigation sidebar and a table of contents that shows the current page headings.
   
   Starlight offers two additional layouts, which can be configured by setting the [`template` field](/reference/frontmatter/#template) in a page’s frontmatter.
   
   The `'splash'` template applies a wider page layout without sidebars.
   This works particularly well for landing pages and you can see it in action on the [homepage of this site](/).
   
   ```md {5}
   ---
   # src/content/docs/index.md
   
   title: My Landing Page
   template: splash
   ---
   ```
   
   The `'blank'` template leaves only the navigation header and creates a blank canvas, which can be useful if you want to fully customize your landing page.
   
   ```md {5}
   ---
   # src/content/docs/index.md
   
   title: Empty Page
   template: blank
   ---
   ```

2. The advantage of documenting the new template there is that we can then just link to it from the changeset and we don't need to document all those details in the changeset, like my suggestion.

With these changes, I think the PR looks pretty solid, and we can wait for maintainers to look at your proposal 👍