Move DDI mods into dedicated folder

[eHanseJoerg]

Description

This Commit moves DDI mods into a new, dedicated folder and creates a _MODIFICATIONS.md file.

Addresses # n/a

Motivation and Context

Cleaning up mods folder.

Type of change

Please delete options that are not relevant.

• [ x] Docs - New (A new document that is neither MCAD or ECAD)
• [ x] Docs - Revision (A revision to an existing document that is neither MCAD or ECAD)

How Has This Been Tested?

not required

Screenshots:

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Checklist: (Delete non-relevant sections)

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