Create glossary of terms

[jmooring]

Closes #2184

[jmooring]

Appearance will improve when gohugoio/gohugoioTheme#267 is merged.

This is mostly accurate, though I did omit some details that site and theme authors really don't need to know (e.g., that a slice is backed by an array).

After going through this exercise, I believe we should stop referring to "site variables", "page variables", etc. Instead, we should call these "site methods" and "page methods".

1. They are not variables. The values are immutable within template actions.
2. Why is .Page.Title a variable but .Page.Pagination a method? It's inconsistent.

I think we should reserve the word "variable" for user-defined variables within template actions... because they are variable.

I suspect the word choices we made 10 years ago were largely influenced by Jekyll.

---

Also, to narrow the usage of "parameters" to site and page parameters, rename the .Params method in shortcode templates to .Args. Functions, methods, and shortcodes take arguments. The site and page have user-defined parameters.

[bep]

I have merged/pushed the theme update, could you rebase this branch?

[jmooring]

I have merged/pushed the theme update, could you rebase this branch?

Thanks for theme merge. This is rebased, and I think I'm done futzing with it.