[DOC] Add conditional execution in pipelines

[vagimeli]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request and all versions that are affected.
Add documentation to close content gap for this topic. Per community feedback, documentation needed for conditional execution in pipelines (nested field conditionals, complex conditionals, conditionals with pipeline processor, conditionals with regular expressions)

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
#4647