Link to C# Variant-compatible section directly

[31]

• After c_sharp_variant.rst: add more examples, fix explicit vs. implicit #8878, the C# variant page's first section is longer. Now the variant compatibility rules aren't necessarily visible when you get there, which could be confusing/annoying.

Added a reference to the variant compatibility rules and linked to it from the top of the page, to start with. Then, I looked for references to doc_c_sharp_variant that seem like they should probably directly point at the variant compatibility section. And I added a brief description to the section to establish some context when you click a direct reference to that section.

In most cases, Variant isn't actually being used--the compatibility rules just have consequences all over the place.

I was tempted to change the section name from Variant-compatible types to Variant-compatible type to avoid a lot of manual reference naming, but resisted it to avoid unnecessary churn and external link breakage.

[raulsntos]

Makes sense to me. Thank you!

[mhilbrunner]

Thanks! Merged.

[mhilbrunner]

Cherry-picked to 4.2 in #9648.