test, [: add page, improve examples (#5205)

pages/common/[.md

@@ -0,0 +1,25 @@
+# [
+
+> Evaluate condition.
+> Returns 0 if the condition evaluates to true, 1 if it evaluates to false.
+> More information: <https://www.gnu.org/software/coreutils/manual/html_node/test-invocation.html>.
+
+- Test if a given variable is equal to a given string:
+
+`[ "{{$MY_VAR}}" == "{{/bin/zsh}}" ]`
+
+- Test if a given variable is empty:
+
+`[ -z "{{$GIT_BRANCH}}" ]`
+
+- Test if a file exists:
+
+`[ -f "{{path/to/file_or_directory}}" ]`
+
+- Test if a directory does not exist:
+
+`[ ! -d "{{path/to/directory}}" ]`
+
+- If-else statement:
+
+`[ {{condition}} ] && {{echo "true"}} || {{echo "false"}}`