Remove link check for dotnet API

[lochhh]

As we have too many references to "https://learn.microsoft.com/dotnet/api/.*", docs build often fails due to 429 Client Error: Too Many Requests for url.

In this PR we configure linkcheck to ignore the dotnet API website.

[glopesdev]

@lochhh yes, this makes complete sense. In fact, I would say we can go even further and remove checks on all links from APIs included in the xref section of the docfx.json file since those are all checked by DocFX when building the site (on top of the ones taken online from the dotnet API).

If it is too complicated we can leave them for now, as they should be small in number compared to the others, but leaving it as a note for future reference.

[lochhh]

@lochhh yes, this makes complete sense. In fact, I would say we can go even further and remove checks on all links from APIs included in the xref section of the docfx.json file since those are all checked by DocFX when building the site (on top of the ones taken online from the dotnet API).

If it is too complicated we can leave them for now, as they should be small in number compared to the others, but leaving it as a note for future reference.

In the PR #49 I have a list of external doc links which are generated alongside the API - I guess we will need to add these as well. But atm, as we're using docfx metadata instead of docfx build, the xrefs are not generated, so there is no need to check these yet (compare for instance "AudioCapture" in Aeon.Acquisition.AeonAudio on our current website vs. on the Doxygen+Breathe preview).