let: add page (#14616)

* let: add page

* Update pages/linux/let.md

Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>

* Update pages/linux/let.md

Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>

* Update pages/linux/let.md

Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>

* Update pages/linux/let.md

* add --help example

Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>

---------

Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>
Co-authored-by: Nelson Figueroa <30811275+nelsonfigueroa@users.noreply.github.com>
Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>

pages/linux/let.md

@@ -0,0 +1,21 @@
+# let
+
+> Evaluate arithmetic expressions in shell.
+> Supports variables, operators, and conditional expressions.
+> More information: <https://manned.org/let>.
+
+- Evaluate a simple arithmetic expression:
+
+`let "{{result = a + b}}"`
+
+- Use post-increment and assignment in an expression:
+
+`let "{{x++}}"`
+
+- Use conditional operator in an expression:
+
+`let "{{result = (x > 10) ? x : 0}}"`
+
+- Display help:
+
+`let --help`