monitoring: rebrand PossibleSolutions as NextSteps

Created by: bobheadxi

As suggested in https://github.com/sourcegraph/sourcegraph/pull/36423#issuecomment-1144405422, this change rebrands PossibleSolutions as NextSteps to lower the bar for this field (the instructions don't need to provide an immediate solution, just guide the next steps once an alert is received)

To accommodate this, the following changes have also been made:

• Refined guidance for how to write NextSteps
• alert_solutions.md -> alerts.md
• prom-wrapper templates now indicate next steps instead of solutions

Test plan

• sg gen go ./monitoring works
• sg run docsite and test that old links like http://localhost:5080/admin/observability/alert_solutions#frontend-99th-percentile-search-request-duration still works