Remove outdated documentation

* The documentation contains mostly outdated and copyrighted information
  taken from Steve Souders's book: `High Performance Web Sites`.

* Removing the documentation doesn't really have any downsides. The rel-
  event information from the docs is already pressent in the `.htaccess`
  file itself, either as inline comments or as links to pages explaining
  the different concepts, or giving more details on the matter.
Improve code consistency
Add link to the MIME Sniffing WHATWG specification
Improve inline comments about `X-XSS-Protection`

Provide more information on how the Cross-Site Scripting (XSS)
filter works by default, and in combination with `X-XSS-Protection`
HTTP header.
Update `README.md`

Include a short description of the project, as well as information on how to use
the configurations and what Apache and browser versions are supported.
Use DENY as the default value for X-Frame-Options

The role of the security related examples is to provide the most secure
way of doing things.

In the case of `X-Frame-Options`, it turns out that invisible site-wide
XSS attacks might bypass the header if it's send with the more tolerant
value of `SAMEORIGIN`.

Ref: https://cure53.de/xfo-clickjacking.pdf