Add text descriptions of index behavior

[mradamcox]

Covering a couple of extra pieces that weren't handled in https://github.com/GeoDaCenter/chicago-environment-explorer/pull/137.

• [ ] Add accordion (or other text box of some sort) with description of how we calculate weights percentages + weightMax

  • This would go on the page where variable weights are assigned, and would give users a bit more context around what the numbers mean on the sliders

• [ ] Add accordion (or other text box of some sort) with text description of how we calculate custom index

  • This would go on the final map page, and be similar to (or the same as) the information that is included in CSV download