Improve media_folder and public_folder documentation

When https://www.netlifycms.org/docs/beta-features/#folder-collections-media-and-public-folder was introduced the main intended use case was to allow saving media files in the same folder as the entry under the constraint different entries don't share media files.
Since then we've added support for specifying per collection and per field `media_folder` and `public_folder`. We also allow `media_folder` to be either a relative path or an absolute path which can cause confusion: https://github.com/netlify/netlify-cms/issues/3654#issuecomment-620423495.
We've also added templating and cascading for `media_folder` and `public_folder` which is not documented at all (https://github.com/netlify/netlify-cms/issues/3670#issuecomment-620422267).
We should document all of these changes and provide examples for each scenario.

### Important
Make it clear to what the `media_folder` is used for (specifying where to save the file in the repo) and what `public_folder` is used for (specifying how to output the file path in the frontmatter).