Restructed for STAC API, `file` and `label` extension improvements

[jamesvrt]

Related Issue(s): Closes #12 Closes #11 Closes #10

Description:

• Restructured STAC to suit STAC API
• Summarised file extension at collection level
• Included label extension, updated examples.

PR checklist:

• [X] Code is formatted (run scripts/format).
• [X] Code lints properly (run scripts/lint).
• [X] Tests pass (run scripts/test).
• [X] Documentation has been updated to reflect changes, if applicable.
• [X] Changes are added to the CHANGELOG.

[cuttlefish]

This looks good. Can we add ItemAssets to the collections?

[cuttlefish]

Can we strip out the commented blocks of code? Are they there for a reason?