i207 Reorder and rewrite chapter 3

[kerimoyle]

Addresses #207

Rendered version available here: https://cellml-specification-dev.readthedocs.io/en/i207_reorder_rewrite_chapter_3/reference/index_sectionC.html

[kerimoyle]

Hey @MichaelClerx! I've put in everything (I think) but in your order.

• The best way to change things will be do change the sectionC_interpretation.inc file, as everything is taken from there.

• sectionC_interpretation.inc is still in the same order as previously, but that's not important because all that's controlled from the index_sectionC.rst file.

• I also haven't changed what would be rendered in the normative only PDF - since it's a subset of the informative html files, it's easier to do that one first, then delete to get to the other instead of vv.

[hsorby]

Closing in favour of #356 targeting the main branch.