convert changelog and generate documentation in markdown format.

[zhumin8]

[updated.]

• removed old CHANGELOG.adoc and replaced with CHANGELOG.md.
• in prep for markdown generation, added first-page.adoc and configuration.adoc (sections copied from index.adoc) so that result full doc can be concatenation of docs.
• produced doc per module to markdown to folder docs/src/main/md/
• in spring-cloud-bus-pubsub.md, replaced spring_cloud_bus_over_pubsub.png with link to github.

Used asciidoctor and pandoc to convert asciidoc to markdown.
Steps to generate the markdown files:

# convert changelog to md
sudo asciidoctor -b docbook CHANGELOG.adoc   
sudo pandoc -f docbook -t gfm CHANGELOG.xml -o CHANGELOG.md --shift-heading-level-by=1


# convert doc to md
cd docs/src/main/md
./convert-from-ascii.sh  

# manual copy title over from index.adoc (due to pandoc settings, this title is not carried over.)

[elefeint]

LGTM, but I think we are going to find it harder to maintain the one single reference documentation file.

Is there any way to have individual per-product files that get combined into a single file with some pre/post processing when publishing?

[zhumin8]

this pr is rebased after #1164 merged in so that title level issues are addressed.
Based on the idea to have individual per-product files, revamped this pr to adding separate markdown docs. and leaving the concatenation of them as step in publishing (will be in.kokoro/publish_javadoc11.sh on #1167 ).
Also updated pr description above to reflect latest changes.

[sonarqubecloud]

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[elefeint]

Looks great.
If the two spring integration documentation parts (pub/sub and storage) become unwieldy in the future, we can split them up then.