[Documentation] Document Object ManagerSniff

[lenaorobei]

Describe why the use of Object Manager is considered as a bad practice.

Acceptance criteria

1. ObjectManagerSniff.md file is created.
2. Explanation about why the use of Object Manager is discouraged.
3. Good and bad code snippets provided.

ExtDN's TemplateObjectManagerSniff.md can be used as a good example.

[lenaorobei]

Closing now, because ObjectManagerSniff was removed from this coding standard causing a lot of core findings.

Look forward to implement this checke with the help of other static tool (not token-based one).

[diazwatson]

Is there any plan to bring this sniff back?