Migrate branch `development-3.x`to Antora

[abelsromero]

This PR is for development-3.x branch

Tried to keep changes to minimal. For example I have kept anchors instead of using navtitle.

In summary this PR contains

• Create Antora descriptors and modules (ROOT & appendices)

• Adjusted docs to start with section level 1

• Fix missing attributes: groovydoc-jvm -> removed, was not used. asciidoctor-docs -> Fixed docs urls that were pointing to old site. Created new asciidoc-url attribute. groovy-dsl-name -> replaced by groovy-dsl-name. kotlin-dsl, asciidoctorj-leanpub-name -> not defined, added text directly since only appears once. plugin-jvm, plugin-base -> content was been copied into partials because is not possible to include contents from outside modules.

• Fix cross references

• Removed pipeline and gradle configurations related to docs generation/publication to gh-pages.

• Other fixes: Removed unnecessary [appendix]

rel #433

[abelsromero]

Docs preview https://deploy-preview-66--asciidoctor-docs.netlify.app/home/

[abelsromero]

Could not reproduce the appveyor error in another Windows

[mojavelinux]

Do we even need the AppVeyor build anymore?

[ysb33r]

Do we even need the AppVeyor build anymore?

No. We can kill it off now.

[abelsromero]

No. We can kill it off now.

Removed.

Also squashed the changes.

[abelsromero]

Ping :%

[abelsromero]

I got permissions to merge, so this is on me now. I need to review what I did and ensure the version version: '3.x-dev' is correct.

[ysb33r]

We are going to 4.x now. Maybe you want to update this.

[ysb33r]

@abelsromero Do you want to modify this to rather merge against development-4.x?

[abelsromero]

Close this, I've been thinking on and off about this and I am inclined to move to a "per branch" model so unify how we have the configuration for other projects. We can discuss this after v4 release.

[ysb33r]

Closed as per @abelsromero