age-keygen: add page; age: update page (#10424)

* age-keygen: add page; age: update page

* age: remove `_` before numbers in placeholders

* age: update `--recipient` example

pages/common/age-keygen.md

@@ -0,0 +1,13 @@
+# age-keygen
+
+> Generate `age` key pairs.
+> See `age` for how to encrypt/decrypt files.
+> More information: <https://manned.org/age-keygen>.
+
+- Generate a key pair, save it to an unencrypted file and print the public key to `stdout`:
+
+`age-keygen --output {{path/to/file}}`
+
+- Convert an identity to a recipient and print the public key to `stdout`:
+
+`age-keygen -y {{path/to/file}}`