Add configuration option for search placeholder text

[uncenter]

I'd like to able to configure/override this "Search by title, content and #tags" text easily in the configuration file.

[rothsandro]

What text would you use? Just to understand your use case.

I think we don't allow any text configuration at the moment (except for user generated content, like groups in the nav). Adding this to the configuration doesn't feel right. I would rather add support for translations in the whole app (but that's a bit more work).

[uncenter]

This is for https://learn-eleventy.pages.dev/, I'd like to make it say "Search for lessons by title or content." which is how I have it on the deployed site. Updating at the minute and trying to avoid customizing .app as much this time.