hg-add: add page (#1789)

pages/common/hg-add.md

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+# hg add
+
+> Adds specified files to the staging area for the next commit in Mercurial.
+
+- Add files or directories to the staging area:
+
+`hg add {{path/to/file}}`
+
+- Add all unstaged files matching a specified pattern:
+
+`hg add --include {{pattern}}`
+
+- Add all unstaged files, excluding those that match a specified pattern:
+
+`hg add --exclude {{pattern}}`
+
+- Recursively add sub-repositories:
+
+`hg add --subrepos`
+
+- Perform a test-run without performing any actions:
+
+`hg add --dry-run`