Adding form for membership application

[MABackoffice]

To simplify the onboarding of new members, it would be good to have a standardized application process.

Anything to add or change?

Application form for organizational membership will follow.

[MABackoffice]

Sorry, here the google form for individual membership application as pdf.
Application for individual membership at Modelica Association.pdf

[dzimmer]

I have now added a section to the association.md in #107 that addresses this issue.

I have tried to formulate it concisely but roughly in line with the bylaws.

Please review and provide feedback.