DOC: Technical Reference: Add table for text formatting (#3976)

Co-authored-by: Dongdong Tian <seisman.info@gmail.com>

Author: yvonnefroehlich

doc/techref/index.md

@@ -11,6 +11,7 @@ visit the {gmt-docs}`GMT Technical Reference <reference.html>`.
 common_parameters.md
 projections.md
 fonts.md
+text_formatting.md
 patterns.md
 encodings.md
 environment_variables.md

doc/techref/text_formatting.md

@@ -0,0 +1,29 @@
+# Text Formatting
+
+The table below shows how text added to a plot, such as text strings added via
+`Figure.text`, plot title, labels of Cartesian axes and colorbars, and legend entries.
+Changing the font as well as its color and size in this way allows to change those
+only for specific characters of a longer text. The supported fonts are listed at
+[](/techref/fonts.md).
+
+| Symbol | Behavior |
+| --- | --- |
+| @%*font*%*TEXT*@%%  | Change the *font* of *TEXT* |
+| @;*color*;*TEXT*@;; | Change the font *color* of *TEXT* |
+| @:*size*:*TEXT*@::  | Change the font *size* of *TEXT* |
+| @-*TEXT*@-          | Toggle subscripts on/off |
+| @+*TEXT*@+          | Toggle superscript on/off |
+| @#*TEXT*@#          | Toggle small caps on/off |
+| @\_*TEXT*@\_        | Toggle underline on/off |
+| @!*CHAR1CHAR2*      | Print two characters on top of each other (composite characters, e.g., overstrike) |
+| @\~*TEXT*@\~        | Toggle Greek letters on/off |
+| @@                  | Print the @ sign |
+| @.                  | Print the ° (degree) symbol |
+| @s                  | Print letter ß |
+| @i                  | Print letter í |
+| @a or @A            | Print letter å or Å |
+| @c or @C            | Print letter ç or Ç |
+| @e or @E            | Print letter æ or Æ |
+| @n or @N            | Print letter ñ or Ñ |
+| @o or @O            | Print letter ø or Ø |
+| @u or @U            | Print letter ü or Ü |