Improve documentation for default global metadata

[a-kriya]

The current documentation for siteUrl does not mention that it's added to global metadata by default, and nor does the Global metadata docs:

https://gridsome.org/docs/config/#siteurl (this does not deep-link on Chromium-based browsers for me)
https://gridsome.org/docs/metadata/

It'd be good to document all the queryable fields of metadata, and mention the utility of siteUrl config field.

Moved from gridsome/gridsome#1294