Includes description about delta_ISA as TLAR

Description

As the discussion about the definition of ISA was quite extensive, I documented the current idea about it.

Related Issue(s)

• Relates to https://git.rwth-aachen.de/unicado/aircraft-design/-/issues/85

Screenshots/Logs

Testing Instructions

See on website https://unicado.pages.rwth-aachen.de/unicado.gitlab.io/tutorials/changing-design-specifications/ at the very bottom

Developer Checklist

• Code has been tested locally and/or in pipeline.