Document server-side bidding without client-side adapters

[muuki88]

Document how to run adapters only server-side without the client-side bid adapter.

This should be an example on the examples page.

Relates to

• Improve client-side footprint for server-side adapters Prebid.js#6361
• Support Prebid Server stored impression scenario Prebid.js#7027

Related Resources

• https://docs.prebid.org/dev-docs/adunit-reference.html#stored-imp ( https://docs.prebid.org/dev-docs/adunit-reference.html#adunitbids )
• https://docs.prebid.org/dev-docs/modules/prebidServer.html#configuration