ember-data landing page: use ember-data-overview module main if present

[runspired]

Results of pairing with @jenweber

This allows the ember-data team to maintain and update the overview for ember-data directly with each release, falling back to a built-in overview for versions for which the library did not yet provide one.

I've configured it to use the main from the module ember-data-overview as opposed to ember-data to ensure that any old releases that had ember-data as a module (current releases do not) won't accidentally trip into this behavior.

We should consider expanding this for the ember-cli and ember.js teams to utilize as well, it would be quite easy to do.

The ember-data-overview page is filtered from the modules that will appear in the sidebar to prevent confusion, since it is already the primary link for the project. This also allows us to re-use ember-data module main as an explainer for that particular package vs for the project as a whole.

[jenweber]

@amyrlam I’m going to set this PR up on the staging env and will let you know when it’s ready! Let me know if you think I should tag someone else for review.

[amyrlam]

thank you for getting this across the finish line!

[amyrlam]

@jenweber Can't think of anyone else to review, but I will xpost in #core-learning for awareness!

[jenweber]

I click tested this in staging, and everything looks good there! Github has some pipeline issues affecting heroku, so this will be deployed after those clear up.