Add mentoring for talks/CFPs workflows to Dev Evangelism handbook

Why is this change being made?

After working with @juliafmorgado on submitting a talk for KubeCon EU, I have established new workflows for mentoring and talks/CFP creation that need to be documented in the Developer Evangelism handbook in https://about.gitlab.com/handbook/marketing/community-relations/developer-evangelism/mentoring-coaching/

This MR adds a new section for talks and CFPs, and also updates the learning project/group with a more detailed workflow.

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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