docs: document error context

[joshuap]

Similar to Ruby:

https://docs.honeybadger.io/lib/ruby/getting-started/adding-context-to-errors/

It looks like the client supports this, but it's not in the README.

[subzero10]

@joshuap I was thinking it’s time to create a dedicated documentation page for .NET in the docs project. The current version, which is extracted from the README file, feels too plain when viewed on the docs site. Let me know if you have any objections to this approach. Otherwise, I’ll go ahead and add a dedicated page the next time I work on documentation.

[joshuap]

@subzero10 yep, I'm good with that. Thanks!