Speed up doc builds with spack list --format=html (#6945)

- Generating the HTML from for >2300 packages from RST in Sphinx seems to
  take forever.

- Add an option to `spack list` to generate straight HTML instead.

- This reduces the doc build time to about a minute (from 5 minutes on a mac laptop).

lib/spack/docs/conf.py

@@ -68,9 +68,10 @@
 #
 # Generate package list using spack command
 #
-with open('package_list.rst', 'w') as plist_file:
+with open('package_list.html', 'w') as plist_file:
     subprocess.Popen(
-        [spack_root + '/bin/spack', 'list', '--format=rst'], stdout=plist_file)
+        [spack_root + '/bin/spack', 'list', '--format=html'],
+        stdout=plist_file)
 
 #
 # Find all the `cmd-spack-*` references and add them to a command index