Update documentation styleguide

[flodolo]

Fixes #292

[Delphine]

Just found a nit, otherwise this seems clear to me.

[flodolo]

@peiying2
Adding you for a pass, since you flagged the document in the first place.

[peiying2]

I'm good with what's already written. After reviewing the corrections in the two docs on mozilla.org that I worked on, maybe we can add these items:

• When to use this is a highlighted text - for anything in GitHub, file/repo/folder names, etc
• When to use this is a highlighted text - for UI strings in Pontoon or is it not as clear cut?
• Cross reference within a doc (under Links section). A special case in bullet # 4 under the [Functionality testing](to get to the language list) section, See detailed steps below is referencing a file name, punctuation is removed and the words are strung together.

[flodolo]

When to use this is a highlighted text - for anything in GitHub, file/repo/folder names, etc
When to use this is a highlighted text - for UI strings in Pontoon or is it not as clear cut?

I'm not sure about this, because we haven't been consistent in existing documents. Also, too much code formatting makes it harder, so we have been mixing that.