nagios, nagios2, nagios3, nagios4: add pages (#17136)

* Create nagios.md

* Update nagios.md

* Update nagios.md

* Create nagios2.md

* Update nagios2.md

* Create nagios3.md

* Create nagios4.md

* Update nagios2.md

* Update nagios3.md

* Update nagios3.md

pages/common/nagios.md

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+# nagios
+
+> Legacy host/service/networking monitoring program.
+> Largely deprecated by `nagios4`.
+> See also: `nagios2`, `nagios3`, `nagios4`.
+> More information: <https://manned.org/nagios>.
+
+- Start `nagios`:
+
+`nagios /etc/nagios/nagios.cfg`
+
+- Start `nagios` in daemon mode:
+
+`nagios -d`
+
+- Start `nagios`, print service check scheduling information to `stdout`, then shutdown:
+
+`nagios -s`
+
+- Verify configuration file:
+
+`nagios -v`

pages/common/nagios2.md

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+# nagios2
+
+> Legacy host/service/networking monitoring program.
+> Largely deprecated by `nagios4`.
+> See also: `nagios`, `nagios3`, `nagios4`.
+> More information: <https://manned.org/nagios>.
+
+- Start `nagios2`:
+
+`nagios2 /etc/nagios2/nagios.cfg`
+
+- Start `nagios2` in daemon mode:
+
+`nagios2 -d`
+
+- Start `nagios2`, print service check scheduling information to `stdout`, then shutdown:
+
+`nagios2 -s`
+
+- Verify configuration file:
+
+`nagios2 -v`

pages/common/nagios3.md

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+# nagios3
+
+> Legacy host/service/networking monitoring program.
+> Largely deprecated by `nagios4`.
+> See also: `nagios`, `nagios2`, `nagios4`.
+> More information: <https://manned.org/nagios>.
+
+- Start `nagios3`:
+
+`nagios3 /etc/nagios3/nagios.cfg`
+
+- Start `nagios3` in daemon mode:
+
+`nagios3 -d`
+
+- Start `nagios3`, print service check scheduling information to `stdout`, then shutdown:
+
+`nagios3 -s`
+
+- Verify configuration file:
+
+`nagios3 -v`

pages/common/nagios4.md

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+# nagios4
+
+> Legacy host/service/networking monitoring program.
+> See also: `nagios`, `nagios2`, `nagios3`.
+> More information: <https://manned.org/nagios>.
+
+- Start `nagios4`:
+
+`nagios4 /etc/nagios4/nagios.cfg`
+
+- Start `nagios4` in daemon mode:
+
+`nagios4 -d`
+
+- Start `nagios4`, print service check scheduling information to `stdout`, then shutdown:
+
+`nagios4 -s`
+
+- Verify configuration file:
+
+`nagios4 -v`