Update contributing guidelines to clarify docs mirroring on sdk repos

[shelbyspees]

This is regarding the change discussed in #472 that's been rolled out across most of the language repos.

I hope most of the friction will be solved by #542, but it'll be helpful to describe how the docs mirroring works in a bit more detail. As a bonus, describing the current process may make it easier to automate mirroring/syncing in the future.

[shelbyspees]

@austinlparker one of my thoughts for this is to add a readme each language's website_docs directory in those repos (and have them mirrored back here), at least so people know whether they're in the right place (i.e. "You know you're in the right place because the directory should be called website_docs" basically). That would involve opening PRs against all the language repos though, so I might hold off on that for now.

Even a readme under content/en/docs/ in here might be a good start.

[shelbyspees]

Oh there's also the contribution guidelines page page that gets published 🤔

It's pretty buried at the bottom of the menu though so idk how many people see it. This is starting to be a lot of pages to update. Let me poke at this a bit.