fix(website): Handle virtual DocTypes webview routing

[cogk]

Virtual DocType × WebsiteGenerator = ❤️

Let's say that we have a virtual DocType Remote Blog Post which fetches data from another website (e.g. Airtable, Google Sheets, Nextcloud), which is also a WebsiteGenerator. The Remote Blog Post has a route field, and a published field.

Because the document does not exist in the database, it's not possible to find it using frappe.db.get_value. Instead, we have to use frappe.get_list (or something better, I'm open to suggestions).

An alternative is to add a get_docname_for_route method to WebsiteGenerator but this feels ugly.

[codecov]

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##           develop   #24445      +/-   ##
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  Branches      6423     6417       -6     
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[cogk]

In this PR I did NOT introduce a controller method to map a route to a document (e.g. get_docname_for_route(route: str)). However, this means that developers have to:

1. Have a well designed get_list controller method.
2. Take care of caching results to avoid unnecessary side effects (network requests, computations).

We might possibly have to consider a refactor of this method to call the hypothetical controller's get_docname_for_route method for fast querying. This means factoring out the whole frappe.db.get_value, leading to:

class WebsiteGenerator:
  @classmethod
  def get_docname_for_route(cls, route: str):
    return frappe.db.get_value(...)

[cogk]

Or maybe developers should just make a page in their www and handle things there. So we should disallow a Virtual DocType to also be a WebsiteGenerator ?

[ankush]

Take care of caching results to avoid unnecessary side effects (network requests, computations).

Framework does it for web view routing. Once route is resolved it stays cached for a while. I'd still avoid this TBH. Just writing plain pages which can use any doctype or external sources achieves the same results (?)