#1056: generate asciidoc to docbook and then docbook to rtf

[hohwille]

This PR fixes #1056.

Implemented changes:

• centralised plugin management
• deleted RTF from git
• convert LICENSE from adoc to docbook
• convert LICENSE from docbook to RTF

ATTENTION: For conversion from docbook to RTF I am using docbkx-maven-plugin.
This plugin and its entire ecosystem (net.sf.xslthl, net.sf.docbook and net.sf.offo) are dead and unmaintained for years (decades).
The process fails with this error:

[ERROR] Failed to execute goal com.agilejava.docbkx:docbkx-maven-plugin:2.0.17:generate-rtf (docbook-to-rtf) on project ide-cli: Execution docbook-to-rtf of goal com.agilejava.docbkx:docbkx-maven-plugin:2.0.17:generate-rtf failed: Cannot invoke "org.apache.fop.fo.pagination.Flow.getFlowName()" because the return value of "org.apache.fop.fo.pagination.PageSequence.getMainFlow()" is null -> [Help 1]

I do not see that this approach will lead to a working solution.
Even creating a big in their issue/tracker on sourceforge seems pointless since nobody will read it...

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[hohwille]

Bug already exists:
https://issues.apache.org/jira/browse/FOP-2182

According maven plugin is more than dead:
https://docbkx-tools.sourceforge.net/source-repository.html

All source links to the code do not exist or work anymore.