RFR: 8206311: Add docs-javase, docs-reference to CI build

[Erik Joelsson]

Reposting this review as PR.

In order to regularly build the javase and reference docs images, I have made the following changes:

1. Introduced bundle targets for the javase and reference docs images.
2. To run this in CI without adding more unnecessary build time to the linux-x64 target (where the current docs are
built), I created a new jib profile "docs". This will run in a separate task and will build all 3 docs targets. 3. To
enable the docs targets to run standalone without having to build a full JDK, I had to tweak the "build-jdk" concept a
bit, to allow it to also function when not cross compiling for certain parts of the build. Specifically the
buildtools-modules can now be built by a "build-jdk" instead of the exploded image. 4. For symmetry, I renamed
variables and targets involving docs images to always include one of "jdk", "javase" or "reference" in the name. See
below for targets.

The new top level targets are:

docs-{jdk.javase,reference}-image:
Aliases for the existing docs-{jdk,javase,reference} added for consistency. The old docs-image is now an alias for
docs-jdk-image.

all-docs-images:
Builds all 3 of the above docs images.

docs-{jdk,javase,reference}-bundles:
Builds the bundles for each kind of docs image. The existing docs-bundles is now an alias for docs-jdk-bundles.

all-docs-bundles:
Builds all the docs bundles.

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Commit messages:
 - Add bundles targets for javase and reference docs

Changes: https://git.openjdk.java.net/jdk/pull/99/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=99&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8206311
  Stats: 142 lines in 7 files changed: 107 ins; 2 del; 33 mod
  Patch: https://git.openjdk.java.net/jdk/pull/99.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/99/head:pull/99

PR: https://git.openjdk.java.net/jdk/pull/99