docs: Tidy "What to look for" section in SC/21.

Removed duplicate use of the chunking box in html.

Moved "Data chunking" text box to its own paragraph and reword
reference to its location, and thus "Quality statistics" section
has text properly merged (a paragraph had started mid sentence).
Adjusted the spacings around the box (different for PDF and html).

Made paragraph headings start on newlines in html.
docs: Better spacing for html in "Observing modes" section of SC/21.
docs: Revise list of graphics files for HTX in SC/21.
docs: Avoid multiple hyperlinks on the same page in SC/21.
docs: Insert space between catalogue name and number in SC/21.

This standard is often forgotten or not realised.
docs: Move a figure for html to agree with the text in SC/21.

Also fix a typo in the caption.
docs: Fix internal cross-referencing in SC/21.

Some had the wrong or mistyped labels.  Some of the htmlonly text
didn't make sense after the section restructuring.  Remove a figure
reference that had been duplicated by me, because the original wasn't
in the correct place.
docs: Fix non-\texttt filenames in SC/21.
docs: Address spacing issues in hypertext SC/21.

Adjust so that the gaps look reasonoable in both PDF and hypertext.
There were some others requiring negative space for the html, which
have yet to be improved.
docs: Turn short subsections into titled paragraphs in SC/21.

This is in the "Writing out models & intermediate maps" section.
In HTML it looked better to have these on one page.
docs: Correct a few more typos in SC/21.
docs: Add two captions missing from the list of figures in SC/21.
docs: Use the \myfig command for skyloop in SC/21.

Also corrected the sense of the caption, wich is contrasting the two
approaches.
docs: Use a better-quality histogram plot for SC/21.
docs: Fill gaps in the dimmconfig tabulation in SC/21.
docs: Major change to the dimmconfig table in SC/21.

It was too cramped, the text too small, and well overran the page
in PDF form.

-  Broke it into three sections, each on a page, and
   with "continued..." at the foot of the first two.
   There seems to be a lot of blank but space on the page with
   the section heading, which forced breaks sooner than desired.
   Thus it required  "...(continued)" headings in the table.

-  Enlarge the text from footnotesize to small, and adjusted the column
   widths.

-  Removed the constricting arraystretch of less than 1.

-  Switched from table* to table as we only have one column.

-  Fix SNR to S/N. [The woman keeps changing her mind!]

-  Added typography for the modes, but not texttt for the config
   parameters in the descriptions.

-  Made consistent over not ending descrptions with a full stop.
   It was inconsistent before.

-  Reworded some descriptions.  Some more could be clearer...

-  Added link to SUN/258 Configuration Paremters appendix.

-  Wrapped the descriptions to avoid long lines while editing,
   and aligned columns.
docs: Adjust section levels in SC/21.

Holly's version had used article style with Chapters, and so the
section levels were one step higher in her version.  During the meld,
especially of longer sections, some of these were missed.

Also the Appendix uses subsections now.