Docs: Update server configs links

* Replace links to the Apache Server Configs `doc`
  directory, as it no longer exists:
  https://github.com/h5bp/server-configs-apache/commit/4f778c4a22da91721bc37c2eb10989b902ac4429.

* Provide missing links under the `X-UA-Compatible`
  section in the `html.md` file.
Docs: Update links and reorganize `extend.md`

* Update various links.
* Remove link to the `Node build script` repository as the project is no
  longer maintained.
* Better organize the information provided in `extend.md` by rearranging
  the sections alphabetically by title, and including a table of content
  for faster navigation.
Update Modernizr to v2.7.1
Update `.htaccess` file
README.md: Add link to the list of releases

Close #1480.
Docs: Update MDN link paths
Docs: Add anchor for MDN link to meta charset
Clarify the default of only 2 icons

In extend.md, which is linked as source for 'more details' on the
touch-icons and favicon, this commit clarifies that by default two
icons are enough if one doesn't want to use art-direction.
Docs: Add comment about `lang` attribute on <html>

We don't set a default `lang` attribute on `<html>` due to the
discussion in #1110. This commit adds a note about how to set it.

Close #1483.
Docs: Make minor improvements in `extend.md`

* Wrap text at 80 characters.
* Rephrase content from the `Apple Touch Icons` section to limit subject
  to only `apple-touch-icon`s (ref: #1486).
* Other improvements.
Docs: Add note about iOS 7.1 `minimal-ui` property

Ref: #1485.
Close: #1485.
Update `CHANGELOG.md`

* Include missing references.
* Improve consistency.
* Wrap text at 80 characters.
Remove default margin from print styles

Providing a default value (such as `5cm`) for the print margins is
problematic because:

 1) in some browsers, it prevents users from customizing the browser
    print settings
 2) in some contexts (browser/os/printer), the value may be too:
    * small, and the margins may collide with all the different
      headers/footers (URL, date, etc) that are usually printed
      by default
    * big, and the margins will just waste the space

Most browsers provide a default non-zero value for the print margins,
so it's better to not specify a value at all, and just let them handle
this themselves.

See also:

 * http://www.w3.org/TR/css3-page/#at-page-rule
 * https://developer.mozilla.org/en-US/docs/Web/CSS/@page

Close #1477.
Docs: Add note about Chrome Mobile web apps

"Add to Homescreen" feature in Chrome Mobile

Closes #1441.
Remove default foreground color from form elements

Specifying a default foreground color for the form elements can be
problematic when users (often with visual impairments) use a darker
OS/browser theme. Also, the color set on the form elements is quite
opinionated, so developers tend to change it anyway.

Ref: #1390.
Close: #1390.
Update `404.html`

* Link to a more official page for the information regarding IE'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 compressed file size even closer to the 512 bytes limit.

* Stats:

  - before:

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

  - after:

     original size:         1245 B
     gzipped size:           582 B
     ─────────────────────────────
     reduction:              663 B [53.3%]
Update to Apache Server Configs v2.1.0
Merge remote-tracking branch 'refs/remotes/html5-boilerplate.git/master'

Conflicts:
	index.html
	sass/_main.scss