Document author

[c-git]

IMPORTANT: Please do not create a Pull Request adding a new feature without discussing it first.

The place to discuss new features is the forum: https://zola.discourse.group/
If you want to add a new feature, please open a thread there first in the feature requests section.

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

This adds author to the page that shows what fields are available in the configuration.

It also explains how the author from the configuration and authors from the front matter are used for feeds.

I'm on the fence about how to add author to the configuration as it is blank and not set to "Unknown" that happens in the feed template. However, the way I've done it causes the config not to work if you just copy and paste it, without editing it. I think that's probably bad but I patterned on the front matter documentation where the date and update are done the same way. I considered commenting out the line but then it's easier to miss. Not sure what is the best way to treat this.

[c-git]

I'm unable to link this PR to the related issue.

[Keats]

Thanks!