Documentation improvement: Clarify best practices

During a recent conversation with @mtruj013 I was told that our documentation is used by sites devs as a source of truth for best practices on using Vanilla. However there are some best practices that are not documented on our site, and are only spread by word-of-mouth through code reviews.

A frequent sticking point in code reviews is the mixing of component class names, like this:

<div class="row p-section"

This makes code a bit harder to read, and also increases changes of issues like this happening.

We should make it clearer in the docs (or somewhere else?) that this is discouraged. There are likely other best practices that we can add there, so this issue can stand as an open discussion of what best practices are currently unwritten and only coming up in code review. This way, our best practices can be codified, rather than existing as "rumors".

canonical / vanilla-framework

Documentation improvement: Clarify best practices #5369