Update source/handbook/product/index.html.md.erb

I found a lot of unnecessary repetition in the last few paragraphs on the Product Page and removed repeating sentences to improve the content flow.

GitLab welcomes and supports new integrations to be created to extend collaborations with other products. GitLab plays well with others by providing APIs for nearly anything you can do within GitLab. GitLab can be a provider of authentication for external applications. GitLab is open source so people are very welcome to add anything that they are missing.

If you don't have time to contribute and are a customer we'll gladly work with you to design the API addition or integration you need.

There is some natural tension between GitLab being a single-application for the entire DevOps lifecycle, and our support for better user experience via integration with existing DevOps tools. We'll prioritize first our efforts to improve the single-application experience, second to enable a rich ecosystem of partners, and third to improve integration with the broader ecosystem to other tools. GitLab is open-source so this should not prohibit contributors adding integrations for anything that they are missing - as long as it fits with GitLab product vision.

And GitLab is open source so people are very welcome to add anything that they are missing. If you don't have time to contribute and are a customer we'll gladly work with you to design the API addition or integration you need.

Wasn't sure whom to assign this to, please let me know if there's a better person to review Handbook related MRs. 🙂