Update compose file versioning documentation

[shin-]

See: #2421 (comment)

## Service configuration reference I would most likely put under this heading a paragraph about the fact there are multiple versions. Then put a cross -reference to the version section to follow.

Then move ## Versioning to follow the main body of configuration material

This section would need to answer:

• define whether the configuration version matches the product version
• under what conditions can I use a different version; for example does the version of the configuration file match or differ from the Compose version
• can I mix syntax in the same config file (prob not but should say)
• can I use two config files that use differing versions of the syntax (I'm thinking also of how the override files interact with the main config under the version conditions)
• provide some guidance to the reader under what circumstances this version mixing is useful

[dnephin]

We'll also need to document the different merging scheme for image and volumes (#2458) as part of this

[dnephin]

We also need to include the build section from #2540