Improve spatial extension docs by linking the extension's own docs

[mwip]

I use the spatial extension a lot. Its current docs already provide a nice overview of the available functions. However, sometimes a bit more detail is required to successfully use its full potential. Somehow by chance I found the documentation provided by the extension itself here. I think these awesome doc files could be advertised a bit more prominently on the "official" DuckDB docs.

The following improvements are possible, ordered by increasing change volume (and possibly maintenance overhead):

1. Link the three spatial docs files (examples.md, functions.md, core.md) in the original docs at appropriate places and explain that more detailed docs can be found there.
2. Link individual chapters, specifically for functions.md.
3. Link individual functions to their more detailed references in functions.md.
4. (??Fully incorporate spatial docs into the duckdb-web repo??)

I would be happy to contribute in any way. Let me know what you think is the best way to do this or whether there are any other plans already.

NB: The current state of the duckdb-web for spatial does not cover all spatial functions listed in functions.md. This probably should be discussed in a different issue, though.