texcount: add page (#10625)

texcount is a command line tool that comes with
TeX Live preinstalled. It counts words in TeX files or
TeX document structures while omitting macros.

Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/common/texcount.md

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+# texcount
+
+> Count words in TeX documents omitting macros.
+> Note: if the TeX document uses `\include` or `\input` and you want to count the included files, `texcount` must be run in the directory of the root TeX file.
+> More information: <https://app.uio.no/ifi/texcount/howto.html>.
+
+- Count words in a TeX file:
+
+`texcount {{path/to/file.tex}}`
+
+- Count words in a document and subdocuments built with `\input` or `\include`:
+
+`texcount -merge {{file.tex}}`
+
+- Count words in a document and subdocuments, listing each file separately (and a total count):
+
+`texcount -inc {{file.tex}}`
+
+- Count words with verbose output:
+
+`texcount -v {{path/to/file.tex}}`