Improve documentation for default global metadata

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_

Start with the `siteUrl` section at https://gridsome.org/docs/config/#siteurl and the Global metadata page at https://gridsome.org/docs/metadata/. Document that `siteUrl` is included in global metadata by default, explain its utility, and list all queryable `metadata` fields; the documentation is complete when both pages cover these details consistently.

Written by the indexing model from the issue text.