Remove Prometheus Monitoring doc

[sophie0730]

What this PR does / why we need it: The monitoring document has already been migrated to the Kubeflow website, we can now remove it from the Training Operator repository.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged): Fixes https://github.com/kubeflow/training-operator/pull/2254

Checklist:

• [ ] Docs included if any changes are user facing

[google-oss-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andreyvelich

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubeflow/training-operator/blob/master/OWNERS)~~ [andreyvelich] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[coveralls]

Pull Request Test Coverage Report for Build 11480416098

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 11471733891:	0.0%
Covered Lines:	77
Relevant Lines:	77

---

💛 - Coveralls