docs(pkgdown): add aria-label to icon-only navbar items (home, github)

pkgdown's accessibility check warned that an icon-only navbar entry lacks an
`aria-label` for screen readers ("Icon fa-home lacks an aria-label"; the message
is throttled to once per 8h so only the first offender printed). Add
`aria-label` to the `home` and `github` components -- the only icon-only navbar
items. `twitter` already had one, and reference/articles/news/network/data
carry visible text, so they need none.

Author: saiemgilani

_pkgdown.yml

@@ -57,6 +57,7 @@ navbar:
       description: Functions to Access Men's Baseball Play by Play Data
       icon: fa-home
       href: index.html
+      aria-label: Home
     reference:
       icon: "fas fa-code"
       text: " Funcs"
@@ -196,6 +197,7 @@ navbar:
     github:
       icon: "fab fa-github fa-lg"
       href: https://github.com/billpetti/baseballr/
+      aria-label: GitHub
     twitter:
       icon: "fa-twitter"
       href: http://twitter.com/BillPetti