WIP: Landing page

[Aariq]

Adds a landing page to the dashboard using markdown. Edit about.md to edit the landing page/tab. Still to do:

• [x] add value boxes somewhere in the page
• [x] example of how to include multiple .md files with Shiny stuff inbetween
• [x] show team how to edit markdown
• [x] Value boxes
  • [x] Most recent temp
  • [x] Year total precip
  • [x] Most recent adjusted temp

[Aariq]

Ok, I think this pull request is ready to hand over to @MJBarrios and @UArizonaCampusLivingLabforGSI. Things to edit:

• app/about.md
• value box creating functions in app/R/value_boxes.R
• possibly app/app.R lines starting with nav_panel("About",, e.g to switch to different value box making functions.

[Aariq]

@MJBarrios can you replace the banner image with a version that doesn't have the text included? I think that's the last thing that needs to be done before this is ready to merge