lex: add examples and add {Free,Net,Open}BSD pages (#11895)

* lex: add examples and add other BSD

* lex: fix OpenBSD man link

* lex: remove unsupported option from OpenBSD

* lex: apply suggestions from code review

Co-authored-by: Juri Dispan <juri.dispan@posteo.net>
Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

* flex, lex: moving flex options to common/flex.md

* lex: mention `lex` alias to `flex`

* Update pages/linux/lex.md

---------

Co-authored-by: Juri Dispan <juri.dispan@posteo.net>
Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/osx/lex.md

@@ -1,17 +0,0 @@
-# lex
-
-> Lexical analyzer generator.
-> Given the specification for a lexical analyzer, generates C code implementing it.
-> More information: <https://keith.github.io/xcode-man-pages/lex.1.html>.
-
-- Generate an analyzer from a Lex file:
-
-`lex {{analyzer.l}}`
-
-- Specify the output file:
-
-`lex {{analyzer.l}} --outfile {{analyzer.c}}`
-
-- Compile a C file generated by Lex:
-
-`cc {{path/to/lex.yy.c}} --output {{executable}}`