d2: add page (#12724)

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>
Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/common/d2.md

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+# d2
+
+> A modern diagram scripting language that turns text to diagrams.
+> More information: <https://d2lang.com/tour/man>.
+
+- Compile and render a D2 source file into an SVG or PNG file:
+
+`d2 {{path/to/input_file.d2}} {{path/to/output_file.svg|path/to/output_file.png}}`
+
+- [w]atch live changes made to a D2 source file in the default web browser:
+
+`d2 --watch {{path/to/input_file.d2}} {{path/to/output_file.svg|path/to/output_file.png}}`
+
+- Format a D2 source file:
+
+`d2 fmt {{path/to/input_file.d2}}`
+
+- List available themes:
+
+`d2 themes`
+
+- Use a different [t]heme for the output file (list available themes first to get the desired `theme_id`):
+
+`d2 --theme {{theme_id}} {{path/to/input_file.d2}} {{path/to/output_file.svg|path/to/output_file.png}}`
+
+- Make rendered diagrams look like hand [s]ketches:
+
+`d2 --sketch true {{path/to/input_file.d2}} {{path/to/output_file.svg|path/to/output_file.png}}`