Update feature-flag-training.md to be more accurate and readable.

Why is this change being made?

Our feature flag training during onboarding is not just about feature flags, it also includes monitoring, debugging, logging, etc...

• Lines were not correctly split, and important notes were not highlighted.
• Sections were mixing feature flags and monitoring.

What are the changes?

• Adds monitoring as a separate section in the .md file -> To have better readability.
• Adjusts sections, to split feature flags and monitoring -> To avoid confusion.
• Adjusts the issue title to include monitoring -> To be more accurate.
• Adds notes in a separate section -> To be highlighted.
• Adjusts the template to be a little bit more readable -> For better readability.

Changes (before/after)

Before

After

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---