[Doc]: Add GitHub Discussions guidelines and category forms

[tomvothecoder]

Describe your documentation update

We've been getting an influx of Discussions forums post which is great. However, we are also noticing that some of these posts are poorly formatted. The original posters are dumping large stack traces with no markdown formatting, making it difficult to parse.

We should also include guidelines for Discussions posts.

• Add GH Discussions category templates
• Add guidelines for posts
• Refer people to xarray/Pangeo forum if it is an xarray question (xarray subsetting, grouping, etc)