Spiff up the 'Developing Plugins' page

* Add more cross-documentation references
* Fix some broken links
* Add links to github/source code references
Add support for Scientific Linux for hostname module

hostname module was lacking support for Scientific Linux, this commit adds it.
Merge pull request #5658 from rishid/devel

Add support for Scientific Linux for hostname module
Removing colon from example failure message

The example failure message was given as - fail: msg="Bailing out: this play requires 'bar'"

The colon in the example causes a YAML compile error, so I am replacing it with a '.'
Merge pull request #5665 from Taytay/devel

Colon in example failure message causes a YAML compile error
Fixes #5661 Handle null values for sysctl parameters
Make synchronize module work better with local transport, fix #5668
Merge pull request #5586 from tbielawa/spiff_up_plugins

Spiff up the 'Developing Plugins' page
fix str format error due to missing '%' in sysctl module
Merge pull request #5672 from simonz05/patch-1

fix str format error due to missing '%' in sysctl module
Merge pull request #5669 from mscherer/fix_synchronize_local

Make synchronize module work better with local transport, fix #5668