add core documentation in website

[flomonster]

• Add a step to the CI that generate core documentation.
  • Use gradle javadoc to generate docs.
• Unzip documentation and add content the content to the branch.

[elzinko]

Détails :

core side

• generate javadoc during core build
• create an artifact from generated doc
• commit artifact to osrd-website master branch

website side

• unzip in static folder
• add javadoc link from hugo cms theme

Thoughts :

• next step is to use package and package registry (or equivalents) for such task.
• possibly commit directly javadoc (without zip) in osrd-website static folder