Tweak to remove unnecessary ' in coordinates index
Added missing "the"
Re-wrote frame documentation
Tweaks to consistent-ify the sgr-example with some recent changes
Typo fix pointed out by @astrofrog
Content fixes suggested by @taldcroft
sgr-example fixes suggested by @taldcroft
fixed section headings in frames.rst
added missing doc link
integrate suggestions from @adrn
Merge pull request #2558 from eteq/frames-ape5-docs

Complete frame docs for coordinates APE5 changes