Improve CI build: create docs, use Maven cache, attach artifacts

[kriegaex]

The GitHub CI build should be improved as follows:

• [x] Use a cache for Maven artifacts so as to speed up the build process and also shorten the build logs. Scrolling through download messages is tedious. Exclude org.aspectj artifacts from caching.
• [x] Add a build step creating docs, using the create-docs profile, which is inactive by default and only used if profile release is active.
• [x] Add build steps attaching docs and main artifacts (runtime, weaver, tools) including binaries, sources and javadocs to the build.

[kriegaex]

Remark: Currently, the build does not create javadocs, so they are not attached either, only sources and binaries. And of course, the extra builds step create and attach docs.

@aclement, do you think we should create javadocs with each build? I think we can still add that later. I am also thinking about future improvements such as segmenting/modularising the build into a little pipeline, simultaneously adding builds for other OS platforms and maybe even other JVMs like Eclipse OpenJ9, see also #61. In order to not make the matrix too big and test all combinations, we could do things like create docs and attach artifacts only once, but run tests on a wider variety of combinations. OTOH, ~14 of ~16 minutes per build are eaten up by the test build step. So we are not saving much time, limiting the matrix for other steps. But the number of attachments will get smaller, see e.g. the list of artifacts in build run 2080974555. Then again, we might want to have just those artifacts, if we ever need to compare them because of hard-to-debug problems.