Clarifying documentation around themes.

[henrifrancois]

• Defined a theme in Zola

• Clarified configuration of themes

• Clarified theme template functionality

• addresses #1532 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?

Code changes

(Delete or ignore this section for documentation changes)

• [ ] Are you doing the PR on the next branch?

If the change is a new feature or adding to/changing an existing one:

• [ ] Have you created/updated the relevant documentation page(s)?

[Keats]

@henrifrancois is that ready to be reviewed?