docs: Revisions for v8 public API changes with the return string form…

[breautek]

…ats of getPicture

Platforms affected

Documentation

Motivation and Context

Added notes mostly in regards to the public API changes for the return format of getPicture.
Also improved documentation surrounding best practices.

Description

• Added notes about return formats in v8+
• Added notes about the return formats used in v7 and earlier
• Removed createFileEntry example, doesn't seem relevant to the camera plugin anymore.
• Added copyTo example, as should be a common pattern starting in v8 moving forward.
• Added upload example, using standard browser APIs.
• Added example of how to render content when data uris used.
• Added example of how to render content when file uri is used, via fileEntry.toURL()
• Added notes indicating that returned file uris are temporary access grants only that typically only survive until the app is closed. For camera captures, this isn't completely true as app cache directory are used and it will survive much longer, but I feel it is easier to treat it as short-lived temporary for the purposes of illustrating the concept in general. (cleanup should be used anyway which will clean up these temporary files)
• Added notes stating that the FILE_URI returns is an implementation detail, and they should not assume it will always be content:// or file://, etc... This is especially true for android devices.

Testing

VSCode markdown preview

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[breautek]

Note that additional documentation will be added from #909 for making WRITE_EXTERNAL_STORAGE optional.