Update the ECA FAQ

The ECA FAQ's appearance and UX is unlike any of our other FAQs. We should make this consistent.

We should also add a question.

On a few occasions, we've received a question along these lines:

When submitting my Eclipse Contributor Agreement, the online form required me to tick Question B even though my contribution was entirely original (Question A). Please note that Question A describes my actual situation; Question B was checked only because the form would not accept the submission otherwise.

The answer is something like:

The ECA is intended to cover all of your contributions for three years, not any single contribution. To that end, we need to you to check all of the boxes to confirm that you understand that the agreement applies to all your future contributions, which may fall into one of those other categories. Note that for each individual contribution one of (a), (b), or (c) must be true, and (d) must always be true.