Establish consistent capitalization

There are inconsistent stylings of **deon** lowercase and **Deon** uppercase throughout the documentation. There should be an agreed-upon styling, and the other way should be fixed.

As for considerations:
- **deon** lowercase will look like the CLI and the logo.
- **Deon** uppercase will look less weird when used in prose, especially at the beginning of a sentence

There are examples of other command-line programs adopting either styling:
- `grep`, `pip`
- Git, Vim, Emacs, Conda

Other option is to use **deon** lowercase everywhere, _except_ when beginning a sentence. **Deon** is uppercase at the start of a sentence. This is kind of a cop-out, which avoids some awkwardness, but ends up with inconsistency.

Personally, I'm in favor of uppercase **Deon**, because I really don't like the weird sentence beginnings. I could also go with the lowercase-except-sentence-beginning.

FWIW, the [Wikipedia Style Guide](https://en.wikipedia.org/wiki/Wikipedia:Manual_of_Style/Trademarks#Trademarks_that_begin_with_a_lowercase_letter) would suggest uppercasing.