Modify old links and remove duplicates in support eng onboarding

This MR mainly has the following changes:

Links Updates:

1. Support Modes of Work: https://about.gitlab.com/handbook/support/workflows/meeting-service-level-objectives.html
2. ZenDesk SLA Settings & Breach Alerts: https://about.gitlab.com/handbook/support/workflows/working-on-tickets.html#understanding-slas

Reordering:

1. Zendesk account setup check has been moved under Stage 0 which deals with general account setup tasks.
2. ChatOps access setup has also been moved to Stage 0.
3. Bamboo HR citizenship check has been moved to Stage 0 as well.

The above changes makes Stage 0 responsible for account and access related tasks, Stage 1 responsible for technical and process related tasks and Stage 3 remains to be for ZenDesk and hands on work.

Removing Duplicates:

1. Removed Bamboo HR citizenship check from Manager pre-task as it is already a part of Stage 0 account task with the manager tagged.
2. Removed the note that this issue is public before Stage 0 as it is already mentioned at the beginning of the issue.
3. Removed reading the Statement of Support as part of Stage 2 as learners are already asked to read it in Stage 1.

Misc:

1. Edited the Zendesk Support search reference link - https://support.zendesk.com/hc/