Add think big information to Release Management page

Why is this change being made?

Add information about the Release Management think big sessions, as per Creating a home for a single source of truth

How will this merge request impact page speed?

No

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

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