Update index.md to add section on requesting non-public information

[mheadd]

Add section for requesting information that is not available publicly, but that is linked to on various pages in the guide.

There are a number of assets linked to and referred to in this guide that are on the TTS Google Drive, and not available publicly. It might be helpful for viewers outside of 18F/TTS (who may be at other Federal agencies) to be able to ask if certain assets currently on Google Drive can be reviewed for possible public release. Identifying assets that are not currently publicly, that might be possible to share with those outside of TTS could significantly increase the value of this guide for helping other agencies.

This would also be consistent with the 18F Open Source policy:

The default position of 18F when developing new projects is to...Develop our work in the open... Publish publicly all source code created or modified by 18F, whether developed in-house by government staff or through contracts negotiated by 18F.

[ninamak]

Great idea! Would first need a process to go along with it, and should be consistent/compatible across 18Fs various guides. @jstrothman Here's the ticket that goes along with the Slack convo (from about a month ago).

[caseywatts]

I was looking at this page earlier today and encountered this private-documents issue too
https://product-guide.18f.gov/define/roadmap/

The documents linked from that page are exactly the sort of thing I'd nominate to be reviewed for public visibility! :)

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Update: I just opened an issue for that page specifically, based on the instructions in this in-progress PR 🎉