Fixed heading sections to use markdown

Noticed that many of the section headers in this doc were using raw
HTML (e.g. <h2>) tags and were therefore duplicating the text in each
of those titles. Fixed to use proper markdown syntax.
Update hdinsight-hadoop-introduction.md

publish date updated
Merge pull request #5499 from squillace/FixDockerPortal

fixing broken links for tomorrow
Merge pull request #5490 from ghogen/vsgetstartedAUX412

Vsgetstarted aux412
update art and text in data retrieval topics
Merge branch 'master' of https://github.com/Azure/azure-content into FromPublicRepo
Merge pull request #5501 from Azure/FromPublicRepo

From public repo
Merge pull request #5502 from Azure/master

master
Merge pull request #5503 from Azure/DailyPub

Daily pub
Merge pull request #5504 from Azure/AUX412

Aux412
Merge pull request #2169 from srkirkland/patch-1

Changing message template to valid JSON
subhead fix
added link to experts webinar
Merge pull request #5509 from Azure/DailyPub

Daily pub
fixed link
Merge pull request #5508 from lizap/rd1023

added link to experts webinar
Merge pull request #5510 from wesmc7777/master

Cleaning up unused art for localization purposes
Merge branch 'master' of https://github.com/Azure/azure-content-pr into FromPrivateRepo
update sa articles
Merge pull request #2178 from Azure/FromPrivateRepo

From private repo