DOP-2662: Use base url for themes

[rayangler]

Staging Links

• mms-cloud
• mms-onprem
• docs-php-library
• docs - v4.2
• guides

Notes

This PR adds the base_url theme to each of the different theme pages. For local testing, I added the following to the property's Makefile:

DOTCOM_BASE_URL=https://www.mongodb.com/docs

## Build with dotcom base url
dotcom-html: export BASE_URL=${DOTCOM_BASE_URL}
dotcom-html:
	rm -rf build/${GIT_BRANCH}
	giza make html

This allows the docs property to be built with an env variable that can be passed to the html theme. html_theme_options in conf.py included the following option:

'base_url': os.getenv('BASE_URL', 'https://docs.mongodb.com'),

[rayangler]

The favicon may be absent on the staging links, but should be available once mongodb.com/docs is launched. This asset is already present in the dotcom prod bucket

[rayangler]

Adding this as the default base_url to prevent breaking staging/deploying of docs if they don't have base_url defined in their html_theme_options