Aggregated javadoc and docs URLs updates (#8171)

* Aggregated javadocs and docs URLs updates

Uptake build-tools 4.0.2
 - Use new helidon-javadoc-maven-pluging to generate aggregated javadocs
 - Use updated sitegen-maven-plugin to use proper SEO friendly URLs

Refactor docs to docs/src/main/asciidoc
 - Added a README to document new steps for building and serving the docs locally

Added a new top-level artifacts helidon-all that is a "bundle" for all Helidon artifacts
 - 1:1 with helidon-bom but as proper dependencies
 - Can be used for build-time tooling (E.g. aggregated javadocs, config docs processor, docker caching etc.)

Fixed various javadoc warnings.

Fixes #7870

Author: romain-grecourt

.github/workflows/validate.yml

@@ -59,7 +59,7 @@ jobs:
       - name: Spotbugs
         run: etc/scripts/spotbugs.sh
   docs:
-    timeout-minutes: 10
+    timeout-minutes: 30
     runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
@@ -70,7 +70,7 @@ jobs:
           java-version: ${{ env.JAVA_VERSION }}
           cache: maven
       - name: Docs
-        run: etc/scripts/site.sh
+        run: etc/scripts/docs.sh
   build:
     timeout-minutes: 60
     strategy: