Update `404.html`

* Link to a more official page for the information regarding Internet
  Explorer's "friendly HTTP error pages", namely:
  http://blogs.msdn.com/b/ieinternals/archive/2010/08/19/http-error-pages-in-internet-explorer.aspx.

* Remove the indentation for the `<head>` and `<body>` tags in order
  to reduce the file size even closer to the 512 bytes limit. This is
  particularly helpful when compression is not enabled.

* Stats:

  - before:

     original size:         1409 B
     gzipped size:           580 B
     ─────────────────────────────
     reduction:              829 B [58.8%]

  - after:

     original size:         1242 B
     gzipped size:           580 B
     ─────────────────────────────
     reduction:              662 B [53.3%]

Ref: #1443.
Update to Apache Server Configs v2.1.0