Website restructure

[bsipocz]

Check the content of User Guide and Examples and move anything relevant to chapter 1/2 to the interactive book, and remove the sections from the website's opening page.

• Top bar: interactive book | book figures | development

• move API to development

• rename "Development" to something else.

cc @connolly

[bsipocz]

How about this opening page? Potentially we could also switch to the hugo theme the other projects use, but this could be deployed in the interim.

[kabartay]

I would remain only skymap for first user attention. Or might be as a light background, depends on what final style is expected.

Also, too much text, badly organized, new lines with 70% empty rest (sentences needs to be reviewed), something might ve itemized.