Adding guidance for referral reach-outs

Why is this change being made?

I regularly receive referrals from team members who've been approached by someone on Linkedin asking to be referred. When that happens I send them this page and this page https://about.gitlab.com/handbook/hiring/interviewing/.

This update adds the guidance from the interviewing page to the referrals page making it easier to find.

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