Create an explicit ad placement

[davidfischer]

This adds the options for projects to opt-in to have an ad placement in a specific spot on their docs by adding a <div id="ethical-ad-placement"></div> in a specific place.

• For projects where we have a revenue share in place -- something I'm trying to grow -- this allows them to put the ads in a better spot and make more money.
• This puts projects in control of where ads go.

Future things not in this PR

• Allow specifying different ad types (eg. text-only, footer ads, etc.). I envision doing this by adding data-ad-type attributes on the <div>. Currently this only does left-navigation ads.

Screenshot

[ericholscher]

This has been on my list for a while, letting folks choose how to integrate the ad 👍

We should include docs on this as part of this PR I think -- though perhaps we want to test it before we publish docs on it?

[davidfischer]

We should include docs on this as part of this PR I think -- though perhaps we want to test it before we publish docs on it?

We definitely need docs on this explicit placement. However, I wouldn't mind running a little test before we publish those docs. Right now, I'm not sure I want projects I'm not talking to to use this yet.

[humitos]

This is a good improvement! I think we should update other places (prospectus, maybe?) where we say that the ads will be at the bottom of the menu list.