Docs tweaks

- Simplify display headings example markup
- Simplify footer styles
- Remove z-indices hacks in the docs
- Remove redundant modal styling in docs
- Use `escape-svg()` to escape docs sidebar chevron
- Simplify sidebar chevron code
- Cleanup masthead css & fix column width between `md` & `lg`
- Easier to download logo's with the `download` attribute.
- Changed some color codes into variables
- Cleanup brands css

site/layouts/partials/footer.html

@@ -1,4 +1,4 @@
-<footer class="bd-footer p-3 p-md-5 mt-5 text-muted">
+<footer class="bd-footer p-3 p-md-5 mt-5 text-muted bg-light text-center text-sm-left">
   <div class="container">
     <ul class="bd-footer-links">
       <li><a href="{{ .Site.Params.repo }}">GitHub</a></li>

site/layouts/partials/home/masthead.html

@@ -4,7 +4,7 @@
       <div class="col-6 mx-auto col-md-4 order-md-2 col-lg-5">
         {{ partial "icons/bootstrap-stack.svg" (dict "class" "img-fluid mb-3 mb-md-0" "width" "512" "height" "430") }}
       </div>
-      <div class="col-md-7 order-md-1 col-lg-6 text-center text-md-left">
+      <div class="col-md-8 order-md-1 col-lg-6 text-center text-md-left">
         <h1 class="mb-3 bd-text-purple-bright">Bootstrap</h1>
         <p class="lead lead-lg">
           Build responsive, mobile-first projects for the web with the world’s most popular open source front-end component library.