Create AGATE documentation

[WilliGautier]

In docs/docs folder, create a agate folder containing:

• agate_api.md (Single line file containing: <redoc src="./openapi.json"/>)
• agate_doc.md (md documentation of the AGATE service)
• openapi.json (the openapi file that will be used with redoc mkdocs plugin to generate api documentation)

The generation of the openapi.json file has to be added to mkDocs.sh following the example of AIRS/FAM/APROC

The new agate documentation needs to be added to the ARLAS Documentation Table of Content