Add category channel option to handbook

Why is this change being made?

This MR adds another section that details what c_ means when prefixed to a slack channel. This allows us to create slack channels for specific categories within a stage group. The reason this is useful is because it allows people who are working closely within one category to communicate specifically about those topics (for example, FE/UX foundations)

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