Add note about prioritizing contributions and link to 'it's impossible to know everything' sub value on Manager of 1 HB page

Why is this change being made?

In one of our latest crucial conversations training and discussions, we talked about how one of the most commonly shared crucial conversation shared at GitLab is struggling to say no.

I'd love feedback on how we might expand the it's impossible to know everything sub-value to include the idea that it is also impossible to contribute to everything.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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• Assign reviewers for this change to the correct DRI(s)
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