Adding units.format.FORMATS

It is a list of formats that can be found by get_format()
Changed documentation link to a hardlink.
Changed the document title of units/format.rst
Removed the paragraph on Unit.to_string
Language tweaks in units format documentation
Better grammar for CHANGES.rst units
Tests for new-style units formatting
[DEV] Quantity formatting shortcut for values.
Docs for new-style Quantity string formatting
Added Quantity format strings to CHANGES.rst
Added a reference for astropy-units-format
Fixed a typo in CHANGES.rst
Improvements to Quantity format documentation.
Quantity formatting unit tests.
Changing units.tests.test_format to not mask builtin format
Using the builtin-format method for test.
fix errors that appeard after rebasing
Update Time documentation to better reflect current precision levels
Clarifying updates about affiliated packages
Re-introduce basic sanity check on slice in Table.__getitem__