fix(openchallenges): move OC API docs URL to app config

[vpchung]

Fixes #2219

Changelog

• Add new variable (API_DOCS_URL) to openchallenges-app env file
• Add new variable (apiDocsUrl) to openchallenges-app config.json and config.template.json
• Update HTML and TS files that display the footer so that the API docs URL used is dependent on where the app is deployed

Preview

Local deployment:

[tschaffter]

I changed the title of the ticket and PR to reflect that the scope is limited to externalizing the API docs URL and not the entire OC web app domain as initially described in the ticket.