Migrate reference documentation to Antora

[pderop]

This PR is first attempt to migrate Reactor Netty 1.2.x documentation to Antora

There are three logical commits in this PR:

• bfb304d: it contains all modifs done in .adoc files, and also contains antora yaml files
• 7cf6e25: contains all adaptation done in various gradle scripts
• 5c90740: contains adaptations done in the GH worfklows.

To test this PR, you can proceed like the following:

Testing without PDF:

• clone this PR
• switch to java 17 (the docs sub project requires java17)
• build the doc and generate it to docs/build/site/index.html:

./gradlew antora

• you can now browse docs/build/site/index.html
• switch to java 8
• deploy reactor-netty docs to your local M2

./gradlew publishToMavenLocal

• now, you can verify the content of ~/.m2/repository/io/projectreactor/netty/reactor-netty/1.2.0-SNAPSHOT/reactor-netty-1.2.0-SNAPSHOT-docs.zip or ~/.m2/repository/io/projectreactor/netty/reactor-netty/1.2.0-SNAPSHOT/reactor-netty-1.2.0-SNAPSHOT-javadoc.jar

Testing with PDF:

As before, PDF are not generated by default in snapshot mode. If you want to also include a PDF in the generated docs zip, then do the following:

• first, you will need to install asciidoctor-pdf command, because the antora pdf assembler requires it (this command is provided when doing "brew install asciidoctor"):

brew install asciidoctor

• switch to java17
• build antora and force PDF generation (in release mode, no need to use -PforcePdf, it's like before):

./gradlew antora -PforcePdf
this will generate  ./docs/build/assembler/reactor-netty/1.2.0/reactor-netty-reference-guide.pdf

• switch to java 8
• deploy reactor-netty docs to your local M2:

./gradlew publishToMavenLocal -PforcePdf

• verify the content of ~/.m2/repository/io/projectreactor/netty/reactor-netty/1.2.0-SNAPSHOT/reactor-netty-1.2.0-SNAPSHOT-docs.zip, which should also include a pdf in the root dir of the zip.

[pderop]

force-pushed for centralizing antora versions in top level build.gradle

[violetagg]

@pderop Nice! Let's merge it!

[pderop]

@violetagg , thank you for your review !