Updated spanish translations

Added notes on every non-translated doc page (Spanish translations)

Removed old `advanced-installation.rst` + unnecessary `empty` file

Added full English copies for currently non-existent Spanish translations

Added files from root folder missing on commit fc277d611370c096deefa6d3e7929b4088bf7d6d

Updated es/contents.rst according to current version
Merge remote-tracking branch 'docs/3.0' into spanish-translation-notes

Conflicts:
	es/index.rst
Merge remote-tracking branch 'docs/3.0' into spanish-translation-notes

Fixed Conflicts:
	es/installation.rst
Merge pull request #1639 from elboletaire/spanish-translation-notes

3.0 Updated cakephp overview + added notes on every non-translated page (spanish translations)