feature: Search Form Block

[jbmoelker]

To do

• model actual block in CMS and use it.
• resolve todo form instance IDs, while maintaining support for deeplinking (SearchLink component).
• remove the hardcoded blocks from the Not Found page.

Changes

• Adds a content block that allows editors to add a search form to any page.
• The search form is connected to the search page.
• The search input is pre-filled with a query from the CMS / search param / path name. See block's README for more details.

Associated issue

Resolves #165

How to test

For this WIP:

1. Open preview link
2. Navigate to a page that does not exist: /en/not-found/anywhere.
3. Verify that the "Search Form block: query from CMS" contains the hardcoded query from 404.astro.
4. Verify that the other search form inputs use the path name (prettified): not found anywhere.
5. Change the URL to include a query search parameter, like: /en/not-found?query=some-text.
6. Verify that the other search form inputs use this query value.

Checklist

• I have performed a self-review of my own code
• I have made sure that my PR is easy to review (not too big, includes comments)
• I have made updated relevant documentation files (in project README, docs/, etc)
• I have added a decision log entry if the change affects the architecture or changes a significant technology
• I have notified a reviewer