Small fixes (#155)

* Fix various typos and references or links

- Misuse of product macro the x sign did not appear
- Use macro for milligrams
- Some refes were without hyperlink for some reason ?
- confusion between section and subsections
- a little grammar.. exact value not absolute, don't think a negative
  amount of flower was ever an option :D

* Use siunitx for temperature everywhere

Apparently some files escaped my script last time, or messed up in git
merge... Anyway fixed now and should be more consistent.

* Simplify Fermentolysis table

I woudl hope this is more clear with less lines.

* Remove double bottomrule

* Fix grammar

* Use correct font familly in preface

Small corrections on font familly and extra space for signature.
TODO: I did not change the paragraph breaking but we might want to
revisit it later.

* Fix wrong percentage range

fixes https://github.com/hendricius/the-sourdough-framework/issues/156

book/tables/table-starter-usage-activity.tex

@@ -1,12 +1,9 @@
-\begin{tabular}{@{}ccr@{}}
+\begin{tabular}{@{}crr@{}}
 \toprule
-                 &\multicolumn{2}{c}{\textbf{Starter}}\\
+                &\multicolumn{2}{c}{\textbf{Amount (\%) of a starter}}\\
                           \cmidrule(rl){2-3}
-\thead{°C / °F} & \thead{Recently fed?} & \thead{Amount (\%)} \\ \midrule
-        30 / 86 & Yes & 5  \\ 
-        25 / 77 & Yes & 10 \\ 
-        20 / 68 & Yes & 15 \\ 
-        30 / 86 & No  & 2.5\\ 
-        25 / 77 & No  & 5  \\ 
-        20 / 68 & No  & 10 \\ \bottomrule
+\thead{°C / °F} & \thead{Recently fed} & \thead{Starving}\\ \midrule
+       30 / 86  &  5 &  2.5 \\ 
+       25 / 77  & 10 &  5   \\ 
+       20 / 68  & 15 & 10   \\ \bottomrule
 \end{tabular}