Consolidate background common to contributors and users

[tjann]

Some of our docs can be combined and reused-- a separate section:

Background
API
Contributing

E.g. we have https://docs.datacommons.org/data_model.html AND a link to schema.org data model at https://docs.datacommons.org/contributing/background/background.html

StatVars are important to users too: https://docs.datacommons.org/api/python/stat_value.html, so information about StatVars (currently only here) will be relevant.

For this issue, feel free to take large freedoms. As Guha put it, the current Representing Statistics documentation is not written with sensitivity to those who have not been in the space for years.