style guide: inconsistent representations of placeholder values

[tallclair]

We should try to be consistent with documentation style, which maybe warrants adding a style guide. For starters, I've noticed inconsistencies in how <placeholder> values are represented:

<placeholder>
PLACEHOLDER
%placeholder%
<compound placeholder>
<compound-placeholder>
<compound_placeholder>
$PLACEHOLDER

Angle brackets <placeholder> seems to be most widely used (recommended by the k8s docs style guide). Capitalization and spacing might be optional, but my personal preference is for upper case with underscores: <COMPOUND_PLACEHOLDER>