Release preparations for Maven Central

[mtf90]

As requested by @fhowar, this PR introduces necessary changes for preparing a RALib release on Maven Central.

It includes

• two new build profiles
  • bundles triggers the building of additional javadoc and source .jars. This profile is also enabled in the CI pipeline to prevent any surprises when performing a release.
  • release triggers additional signing of the built .jars and enables Sonatype's central-publishing-maven-plugin.
• a configuration of Maven's release-plugin to automate the release process (tagging, bumping versions, etc.).
• some meta-data to enrich the project description.
• some fixes to previous javadoc errors.
• some ignores to IntelliJ's project files.

What this PR does not (yet) include is a thorough consolidation whether the current state is ready for release. A few questions that came across my mind during preparation were

• is the developer/contributor information correct/complete?
• do you want to release an artifact relying on preview compiler features (cf. --enable-preview flag)?
• do you want to release RALib under <groupId>de.learnlib</groupId>? This should still be distinguishable from (regular) LearnLib as its artifact IDs usually use the learnlib- prefix, but a distinct groupId like de.learnlib.ralib (similar to what ALEX uses) should work, too.
• is there any branching model that we should follow or is it enough to just release from the (stable) main branch?

These questions may be answered in separate PRs.

[mtf90]

The point about preview features is obsolete once #113 is merged.