Examples fixes and preview.astro.new support

[delucis]

Changes

• This PR (partially) fixes support for base in the blog, portfolio, and with-nanostores examples.
  • In the blog and portfolio examples, this fixes the highlighting of the current page in the site navigation components. It doesn’t fix other aspects of these templates to work with base (e.g. image paths etc.) just for simplicity’s sake, but we could in a follow up if we wanted.
  • In the with-nanostores example, this fixes all user-authored links to use a base.
• This improves the deployments to preview.astro.new, which builds each example with base and then tries to auto-prefix with base when it can, but there are some edge cases it can’t handle, which this PR fixes
• This PR also adds a new GitHub workflow which runs when pushing to main. It checks to see if anything in the examples/ directory has changed, and if so sends a request to Netlify to redeploy preview.astro.new
• This workflow requires a new NETLIFY_PREVIEWS_BUILD_HOOK secret adding to this repo. @ematipico added this ✅

Testing

• Ran the examples locally with and without base set

Docs

n/a bug fix only

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[ematipico]

I didn't checkout the branch to check the UI changes. I left a couple of suggestions for the workflow.