How to handle in SpaceCat sites where only some of the pages/subtrees are migrated to aem_edge?

## Overview
How to handle in SpaceCat sites where only some of the pages/subtrees are migrated to aem_edge?

## Details
There are customers where only a set of distinct pages or only a subtree of the sites is migrated to aem_edge.
There are customers that "go live" when this first set of pages or subtree is migrated, while the migration continues in batches of pages during "post go-live".
This leads to confusion in SpaceCat because we only have the concept of a site that:
* has a single baseURL
* can be live or not
* of a specific delivery type or other

Example:
https://cq-dev.slack.com/archives/C05B53SRX7V/p1710264532174059
They only migrated 2 pages, but:
* the baseURL is the whole domain, even if only 2 pages from the domain are actually of interest atm
* the delivery type is aem_edge
* isLive is true and toggled at 2024-04-05

This leads to increased complexity and potential confusion when running SEO assessments (sitemap, broken backlinks etc.), because those audits consider the whole subtree of the baseURL as pages/sitemap of interest.

## Proposed Actions
0. Improve SpaceCat documentation/slack command to add site so that it's clear that in case of a subtree, that's the one that should be added (in case of a subset of pages, the closest would be the path representing the subtree intended to be migrated)
1. Improve site discovery to take into account first the subtree case (more common), then if possible at all the set of pages?
2. Improve SpaceCat data model to accommodate these cases