docs: Add salary adjustment instructions

[RangerMauve]

BAsed on last weeks ops meeting.

Questions:

• Should we link directly to the sheets?
• Sheets have instructions for payroll and stuff, should we extract that to the handbook?

[fastfadingviolets]

IMO let's keep this separate from the sheets. The sheets feel a little more inside baseball to me (or maybe need cleanup to be public)

[LexaMichaelides]

I'm fine with referencing the sheets if we specify that we have a spreadsheet and the instructions might not make full sense to an outsider. But should change the 2.5% to our "vibes based" policy referenced in the decision log