Analytics for downloadable files (PDFs, Word, etc)

### As an austin.gov team member, I want to know how often users access documents, so that I can make better product and content decisions going forward.

**Is your feature request related to a problem? Please describe.**
We now host a number of documents on our website on pages like the OPO's [official disciplinary documents](https://alpha.austin.gov/en/police-oversight/official-complaint-and-discipline-documents/) and [reports and recommendations](https://alpha.austin.gov/police-oversight/office-of-police-oversight-official-reports/), as well as [documents related to mobile food vendor permitting](https://alpha.austin.gov/en/health-safety/health-prevention/disease-prevention/documents-for-mobile-food-vendors-in-austin/). However, Google Analytics isn't set up to track the usage for these.

[Google Analytics support on setting up tracking for downloadable files](https://support.google.com/analytics/answer/1012044?hl=en)—there's also a link for analytics developers.

**Acceptance criteria**
- [ ] Google Analytics tracks every time a document is downloaded
- [ ] A user in Google Analytics can view by type of documents (PDF, Word, etc)
What else for acceptance criteria?

We might want to compare with how austintexas.gov has document tracking set up, and make sure we're matching or exceeding their setup.