Add repo jbake-maven-plugin into repo jbake + upgrade to gradle 7

[ge0ffrey]

• Create subdirectory jbake-maven-plugin with the content of the old repo
• Don't lose git history or attribution
• Build maven plugin as part of the build
• Upgrade to Gradle 7.x to the previous to work (Thanks Ancho!)
• Documentation and release notes - PR for that: jbake-maven-plugin is now part of the jbake release lifecycle jbake.org#150
• Example using the plugin (proof that it works well too) - PR for that: Add maven pom [after 2.7.0 is released with PR 740] jbake-example-project-freemarker#14

Tackles these issues:

1. A JBake release should automatically release the maven and gradle plugin too. More efficient releases.
2. The version number of the maven and gradle plugins should match the JBake versions.
   Keep it simple for users: If a new JBake version comes out, I just update my jbake-maven-plugin version, done. No need to figure out which maven/gradle plugin version aligns to which jbake version (for every jbake upgrade too).
3. The Maven and Gradle versions should use the same asciidoc, markdown, freemarker, etc versions as jbake-core. They should include them all out of the box, so a user doesn't have to explicitly add them in their build files. They shouldn't have to figure out the right asciidoc and freemarker version to use, nor go through all that again every time when upgrading to a new jbake version. The should automatically use the same asciidoc/markdown version as jbake-core unit tests did.
4. All of this begins with moving the maven and gradle repositories into the gradle repository without loosing git history.

[jonbullock]

Thanks @lefou for taking the time to share your thoughts on this, it is very much appreciated. 👍

There are negatives, as you quite rightly point out, but I think the potential positives will outweigh the negatives overall. Especially for users and that is the goal for this.

[jonbullock]

Conducted some local testing and the plugin builds and executes as expected in a sample Maven project. I compared the generated artefact to a previous version and noticed a few things. The pom.xml for the plugin is missing metadata about the plugin (name, license, SCM etc.) and a HelpMojo class is missing (guessing this was automatically generated).

Couldn't remember if the metadata is required for Maven Central so did a check and found this: https://central.sonatype.org/publish/requirements/ which suggests it is.

[ancho]

Have a look at the latest commit here https://github.com/ancho/jbake/tree/merge-repo-jbake-maven-plugin
I had to switch the maven development plugin to make it work.

[jonbullock]

Thanks @ancho that's solved it, with that commit I get the metadata and the HelpMojo class.

[ge0ffrey]

Thanks @ancho! Great work!
@jonbullock I fast forwarded this PR to include ancho's last commit too.
(I don't mind if you merge this branch or your local stuff and just close this one - as you see fit)

[jonbullock]

Thanks so much @ge0ffrey and @ancho