Add API doc summaries for namespaces

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**Is your feature request related to a problem? Please describe.**
We used to have namespace summaries that included the tables of operations/functions/UDTs defined by that namespace and their one-sentence summaries (the first element of their API docs). https://learn.microsoft.com/en-us/qsharp/api/qsharp/microsoft.quantum.arrays is an example URL (redirects elsewhere now).

These were very useful for somebody trying to figure out whether a certain functionality exists within a library, for example, a certain array manipulation logic, but not knowing how exactly it is called. Now, one can only scroll the list of names, and to get the idea of what the operation does they need to go to the specific operation page. This is inconvenient.

**Describe the solution you'd like**
Return namespace summary doc pages.

No files or tests are named. Start by tracing how API documentation pages are generated and compare the current namespace pages with the linked Microsoft Quantum example. Done means namespace pages again include tables of operations, functions, and UDTs with their one-sentence API summaries.

Written by the indexing model from the issue text.