unison: add page (#3358)

pages/common/unison.md

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+# unison
+
+> Bidirectional file synchronisation tool.
+> More information: <https://www.cis.upenn.edu/~bcpierce/unison/download/releases/stable/unison-manual.html>.
+
+- Sync two directories (creates log first time these two directories are synchronised):
+
+`unison {{path/to/directory_1}} {{path/to/directory_2}}`
+
+- Automatically accept the (non-conflicting) defaults:
+
+`unison {{path/to/directory_1}} {{path/to/directory_2}} -auto`
+
+- Ignore some files using a pattern:
+
+`unison {{path/to/directory_1}} {{path/to/directory_2}} -ignore {{pattern}}`
+
+- Show documentation:
+
+`unison -doc {{topics}}`