* Move the generation into sphinext, similar to other overviews we have.
* Generate a table instead of the plain list. This table contains links
  to the lexer class now.

This commit adds a new url field to a lexer, which can be used to link
to the language website, instead of relying on having the link in either
languages.rst or the docstring of the lexer. Additionally, it changes the
languages.rst file to auto-generate the list of lexers from the actual
source code, using the provided URL.

* "Try out Pygments!" is more descriptive than "Try it out!"
* Moved the description of the implementation to the end.
* Added a <noscript> tag suggesting `pip install` and pygmentize.
* Added a <label> for the textarea.
* Disable spell checking for the textarea.
* Reduced the scope of the reset button to the file input.
* Added autofocus for the language select.