Central browser support

This issue is to document which browsers ODK Central supports. We target the following browsers in our build:

last 2 Chrome versions
last 2 Edge versions
last 2 Firefox versions
last 2 Safari versions

Start by locating the relevant page in the repository's Sphinx documentation and review nearby browser or build-support guidance. Document that Central targets the last two versions of Chrome, Edge, Firefox, and Safari; done means the support policy is clearly published in the appropriate documentation location.

Written by the indexing model from the issue text.