This patch modifies the GenMarkdownCustom to skip writing a Synopsis
header with the `cmd.Short` duplicated both before and after the header.
Instead, it only writes the `### Synopsis` header and the paragraph when
a `cmd.Long` has some kind of content in it.
Adds `TestGenMdDocWithNoLongOrSynopsis` as the test case.
Signed-off-by: Marc Lopez <marc5.12@outlook.com>
* Move man_docs and md_docs into new doc pkg
* Replace *bytes.Buffer with io.Writer
* Replace c == cmd.helpCommand with c.IsHelpCommand()
* Remove redundant len(children) == 0 check in HasSeeAlso
* Duplicate test setup for doc generation
