added instruction on how to enable file upload

[armedi]

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[apollo-cla]

@armedi: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[jpvajda]

@brainkim @benjamn I wanted to get your thoughts on this addition to the docs to add a section for Enabling file uploads in Apollo Client the content looks good but it's failing some CI checks, so I was curious if we've seen this issue before on new pages being added to the documentation.

[jpvajda]

Thanks for this contribution! I've made some updates to the original content in a new PR #9744, so I'm going to close this and work from a new PR.