"Welcome to Q&A for ..." or "Welcome to a Q&A site for ..."?

• Wikipedia defines Q&A as an abbreviation for "Questions and Answers".
• Dictionary.com defines it as a stand-alone noun meaning "an exchange of questions and answers".

With either of these definitions, "Welcome to Q&A for..." makes grammatical sense.

"Welcome to a Q&A site for..." would fit the possibly alternate definition of "Question and Answer" that @Brendon Stanton pointed out, but it's a bit wordy. For a tagline, I would probably stick to the shorter version.

I think either would be correct.

The first welcomes people to the questions and answers housed on the site, while the second welcomes people to the site, which just happens to contain questions and answers.

I have no basis, but neither sounds like especially good phrasing to me. It depends greatly on what you take the meaning of "Q&A" to be. I personally always read the abbreviation to myself as "question and answer." This would make the first one incorrect, because "welcome to question and answer for..." lacks an article and disagrees in number with whatever would likely follow.

If you have to choose one, I would pick the second. It is a bit verbose, but there is no grammatical invalidity associated with it.