Clear root files and folders from docs gh-pages branch

[tiagoafpereira]

The current contents of gh-pages branch is:

.
├── api_reference
├── cli
├── cli.html
├── CNAME
├── computational_resources
├── en
│   ├── development
│   └── latest
├── explore_api
├── first_example.html
├── genindex.html
├── get_started
├── Home.html
├── how_to
├── index.html
├── installation.html
├── introduction
├── markdown_sample.html
├── objects.inv
├── search.html
├── searchindex.js
├── simulators
├── _sources
├── _static
├── task_state_diagram.html
└── uninstall_inductiva.html

When it should be this instead:

.
├── CNAME
├── en
│   ├── development
│   └── latest
└── index.html

From #1240 onwards /en folder contains the latest and development version of the documentation and all previous content has become stale and should be removed.