Fix comment in index.html about apple-touch-icons

Close #1438.
Docs: Add info about how to block Google Translate

Inform users on how to disable the translation prompt in Chrome or
block Google Translate from translating a web page or any particular
section of the web page.

See also: https://support.google.com/translate/?hl=en#2641276.

Close: #1440.
Redesign 404 page

Changes:

* simplify the overall design
* remove the Google search functionality as in most cases, it does not
  provide much value
* reduce the overall file size while keeping the gzipped size over 512
  bytes to prevent IE from displaying its own custom error page (thanks
  @mikealmond and @patrickkettner):
  http://www.404-error-page.com/404-error-page-too-short-problem-microsoft-ie.shtml

Benefits include:

* small file size (1409 bytes / 580 bytes gzipped), which is particular-
  ly useful when users are on a data plan and / or the server is wrongly
  configured (e.g.: tries to redirect to a "mobile" version of the web
  page and ends up serving a 404 page) or developers forget to provide
  certain files (e.g.: /favicon.ico -
  http://zoompf.com/blog/2012/04/instagram-and-optimizing-favicons)
* improved performance (less HTTP requests, less memory consumption,
  faster rendering, etc.)
* designed to work with all kinds of screen sizes and support IE 6+

Close #1443.
Add vertical centering for `svg`

Remove gap between the `svg` elements and the bottom of their containers
by vertically centering them.

Ref #1326.
Close #1453.
Docs: Remove outdated info on Skype highlighting

The snippet provided in the docs about how to prevent the Skype
plugin from formatting numbers on a web pages is outdated and no
longer works.

Nowadays, the Skype plugin makes it more difficult to overwrite
its injected styles (class names include timestamps, e.g.:
`skype_pnh_print_container_1381964168`, `!important` is used, etc.).

Providing a solution for this use case isn't viable. Also, users
that install such a plugin usually expect the behavior.

Close #1454.
Fix typo in header of doc/html.md (promt → prompt)

Close #1459.
Fix typo in doc/css.md
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
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`

* 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.