NIFI-8648 Add Session Affinity section to Admin Guide

[exceptionfactory]

Summary

NIFI-8648 Adds a section on Session Affinity requirements under the Proxy Configuration section of the Administrator's Guide. The section provides some background on the reasons for requiring session affinity, as well as links to configuration details for Apache HTTP Server, Nginx, and the Kubernetes Nginx Ingress Controller.

Tracking

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Issue Tracking

• Apache NiFi Jira issue created

Pull Request Tracking

• Pull Request title starts with Apache NiFi Jira issue number, such as NIFI-00000
• Pull Request commit message starts with Apache NiFi Jira issue number, as such NIFI-00000

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Build

• Build completed using mvn clean install -P contrib-check
  • JDK 8
  • JDK 11
  • JDK 17

Licensing

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Documentation

• Documentation formatting appears as expected in rendered files

[ChrisSamo632]

These changes LGTM 👍

Although I can't currently merge due to computer problems

[exceptionfactory]

Thanks for the review @ChrisSamo632!