content/static: improve alt text for image tags in header/footer

This change modifies alt text for the header and footer logos. The goal
of this change is to make the alt text more descriptive of the image
being shown on the page.

Fixes golang/go#41104

Change-Id: Ifda884be828a12712b4c7741ea646d3ef9e58cb5
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/275938
Reviewed-by: Julie Qiu <julie@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

content/static/html/base.tmpl

@@ -35,7 +35,7 @@
   <div class="Header">
     <nav class="Header-nav">
       <a href="https://go.dev/" class="Header-logoLink">
-        <img class="Header-logo" src="/static/img/go-logo-white.svg" alt="Go">
+        <img class="Header-logo" src="/static/img/go-logo-white.svg" alt="Link to Go homepage">
       </a>
       {{template "header_search" .}}
       <ul class="Header-menu">
@@ -168,7 +168,7 @@
   <div class="Footer">
     <div class="Container Container--fullBleed">
       <div class="Footer-bottom">
-        <img class="Footer-gopher" loading="lazy" src="/static/img/pilot-bust.svg" alt="The Go Gopher">
+        <img class="Footer-gopher" loading="lazy" src="/static/img/pilot-bust.svg" alt="Gopher in flight goggles">
         <ul class="Footer-listRow">
           <li class="Footer-listItem"><a href="https://go.dev/copyright">Copyright</a></li>
           <li class="Footer-listItem"><a href="https://go.dev/tos">Terms of Service</a></li>