userguide/content/en/docs: rename mixed-case directory names and names containing spaces

[chalin]

It's usually a good idea to avoid mixed-case directory names and names containing spaces for content sections (or just in general for code or doc related projecqts). All of the directories under https://github.com/google/docsy/tree/master/userguide/content/en/docs fall into this category.

I propose that we rename these to correspond to the section-path names used by the deployed site. For example, rename

• userguide/content/en/docs/Adding content to
• userguide/content/en/docs/adding-content, which corresponds to https://www.docsy.dev/docs/adding-content/

Thoughts @emckean @LisaFC?

• [x] Adding content - #1129
• [x] Get started - #1118

[emckean]

SGTM!