Fix the guide menu and missing index page

[gcangioli]

• [x] most of the menu item (pages) are missing
• [x] the structure doesn't correspond to the propose one
• [x] the index.html page is missing

[gcangioli]

First proposal in https://github.com/hl7-eu/gravitate-health/commit/8917665a5ca3b778d39ad29aa325d1a25a31995f

[gcangioli]

The structure of the guide (as FHIR IG) and of the included pages has to be further discussed

[cander2]

ok, happy to change it to comply. I'll schedule quick time to chat about how to do it right

[cander2]

Do you have an example IG that you think shows the ideal format? We can then adapt everything to follow that

[cander2]

Updated the menus