Update sales rep certification index.html.md

Why is this change being made?

• To add details in the handbook about the sales rep certification process and timelines

Describe in detail answer to question on why this change is being proposed, in accordance with our value of Transparency

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Does this MR meet the acceptance criteria?

Conformity

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