Add resources doc for .NET

[cartermp]

Contributes to #1773

Similar to the JS one, but with some notable changes:

• No process detection section (can add later if there's a good way to do it)
• Code sample is different, as there is no semantic attributes package for .NET yet
• Elided adding docker resources, but could add later if there's a package for that

[pellared]

Just left a comment regarding more options on how to set service.name as it is the only required resource attribute.

[svrnm]

Elided adding docker resources, but could add later if there's a package for that

@swetharavichandrancisco created an extension for that: https://github.com/open-telemetry/opentelemetry-dotnet-contrib/tree/main/src/OpenTelemetry.Extensions.Docker -- does it have all we need?

#1795 (comment)

[cartermp]

@pellared @cijothomas PTAL! Should be all addressed.

[svrnm]

@cartermp, thoughts on adding a section for docker container id with https://github.com/open-telemetry/opentelemetry-dotnet-contrib/tree/main/src/OpenTelemetry.Extensions.Docker ? We can do this as a follow up.

[pellared]

👍