Add 'Understanding and applying MECC' to MECC Homepage and pillar snippets

Following the June 2022 e-group Content Discussion on MECC, a number of content iterations were proposed.

This merge request adds the following:

1. Narrative snippets beneath the four key pillars to add context while viewing the homepage
2. An Understanding and applying MECC sub-head which clarifies that MECC describes an ideal state and MECC is for individual contributors and people managers.

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.

---