docs: Move the section titlesec until the main body of SC/21.

This is to prevent the chapter-style section headings to be used for
the List of Figures and Contents pages, which included the section
number, i.e. 0.  Make the title take the form "Chapter 1: ".

Also remove an trial colour, now unused, for the blue section banners.
Add some commentary so next time we'll have an idea what is being done
and why.
docs: Fix an SC/21 chapter title missed in a308644167065.
docs: Use two smaller graphics files in SC/21.

The size scaling does not work for some of the PNGs in HTML version
(hence not using \htmlonly for those).  Since some of the recent additions
are too big to fit on a typical browser window, reduce them.  We did
have smaller versions last time around...

There is still one large graphic remaining---the STC region outlines---but
the outlines are broken in the smaller graphic.
docs: Move "Quality statistics" in SC/21.

This was needed because the blue caption bands led to the "Data
Chunking" no longing appearing in its own section and leaving
a gap after that section.
docs: Restore the two examples sections in Chapter 8 of SC/21.

Also uncomment the sentence that referenced one of these.
docs: Move jcmtstate remarks into TIP boxes in SC/21.
docs: Complete sentences end in fullstop in SC/21.
docs: Reduce usage of em-dash in SC/21.
docs: Remove dimmconfig_bright.lis from a table and an appendix of SC/21.
docs: Update references in SC/21 to the now published SCUBA-2 papers.
docs: Minor rewording in the ORAC-DR section of SC/21.

It ncludes the command to initialise the pipeline.
docs: Miscellaneous text improvements in SC/21.
docs: Align recent comments in the SC/21 prologue.
docs: Provide links and DOIs for the three SCUBA-2 MNRAS papers referenced in SC/21.