[website] add link to `infer help` man page

Summary:
The page itself was already there but not linked from the rest of the
website.

Reviewed By: ezgicicek

Differential Revision: D21997950

fbshipit-source-id: 30380da65

website/docs/01-man-pages.md

@@ -9,6 +9,7 @@ Here are the man pages for all the infer commands:
 - [infer analyze](/man/next/infer-analyze.1.html)
 - [infer capture](/man/next/infer-capture.1.html)
 - [infer compile](/man/next/infer-compile.1.html)
+- [infer help](/man/next/infer-help.1.html)
 - [infer explore](/man/next/infer-explore.1.html)
 - [infer report](/man/next/infer-report.1.html)
 - [infer reportdiff](/man/next/infer-reportdiff.1.html)