Improve Horizon Runbook

[aditya1702]

Our current Horizon Runbook is in an internal repository and is hard to find. Moreover, it seems to be outdated and has not been updated in a while. This can be a problem for future incidents where we do not have a centralised place to look up some common solutions/fixes and reach out to partners. The following tasks can be tackled as part of this ticket:

### Tasks
- [ ] Review and update the current runbook - one idea is to add links to all previous incidents and their post-mortem reports.
- [ ] Create an incident response/short release document so that partners and team members can reference it and coordinate a hotfix.
- [x] Add a section of current Horizon partners, their infrastructure setup and best way to reach out to them - #4962, #5048

[sydneynotthecity]

Internal runbook is hard to find: https://github.com/stellar/knowledge/blob/master/horizon/oncall.md

Can we create an incident response, or shortened release document so that partners and newer team members can coordinate a hotfix when horizon needs patching?

Runbook is also outdated and should be reviewed and updated.