Fix broken link in Prometheus exporter README. Fixes #4399

[yrsuthari]

Description

This PR fixes the broken link in the Prometheus exporter README.rst by updating it to point to the current documentation location on opentelemetry.io.

The old link pointed to a non-existent page on readthedocs:
https://opentelemetry-python.readthedocs.io/en/latest/exporter/prometheus/prometheus.html

The new link points to the current documentation:
https://opentelemetry.io/docs/instrumentation/python/exporters/#prometheus

Fixes #4399

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Verified that the new link is accessible and points to the correct documentation
• Checked that the RST formatting is correct in the README file

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: emdneto / name: Emídio Neto (3143d11)
• ✅ login: yrsuthari / name: Yugandhar (ba0885b, cae1d02)
• ✅ login: xrmx / name: Riccardo Magliocchetti (82d58ab, 623ef54, fb44405, 38ebaa6, 6941c15)

[emdneto]

I think we are still missing the prometheus-exporter docs published on the opentelemetry-python readthedocs

[yrsuthari]

I think we are still missing the prometheus-exporter docs published on the opentelemetry-python readthedocs

Could you please take a look now @emdneto

[emdneto]

LGTM when docs CI gets fixed. I think it's just a matter of adding the prometheus_client dependency in tox