Add Contrib repo checkout for docs build

[NathanielRN]

Description

This fixes the issue with the docs build which is failing because the Contrib repo is not checked out as part of the docs.yml workflow.

The failure can be seen here: https://github.com/open-telemetry/opentelemetry-python/runs/1379941084

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

No tests, but the errors logs say it's because the opentelemetry-python-contrib repo directory does not exist which makes sense.

Checklist:

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

[lzchen]

Curious as to how the docs build was working before?

[NathanielRN]

@lzchen I think it is because we never triggered the workflow run conditions in docs.yml until recently, when a commit was pushed to master that affected a certain path used by docs.

See:

opentelemetry-python/.github/workflows/docs.yml

Lines 3 to 12 in 26bf23f

	on:
	push:
	branches:
	- master
	paths:
	- 'docs/**'
	- 'exporter/**'
	- 'instrumentation/**'
	- 'opentelemetry-python/opentelemetry-api/src/opentelemetry/**'
	- 'opentelemetry-python/opentelemetry-sdk/src/opentelemetry/sdk/**'