change openms/openms-docs default branch to develop

[tapaswenipathak]

We should change the default branch to develop from staging in sync with OpenMS/OpenMS.

User facing URLs:

• openms.readthedocs.io/en/develop
• openms.readthedocs.io/en/latest

URLs having redirects:

• openms.readthedocs.io/en/staging to openms.readthedocs.io/en/develop
• openms.readthedocs.io/en/test to openms.readthedocs.io/en/develop
• staging-openms.readthedocs.io/en/latest to openms.readthedocs.io/en/develop

Internal development URLs

• staging-openms.readthedocs.io/en/staging

Internal trial URLs

• staging-openms.readthedocs.io/en/test