Use pegdown to generate some documentation files [LUCENE-6195]

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Spinning this out of SOLR-6870 for Uwe's request to do a similar pegdown'ing of Lucene's site files.

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Migrated from LUCENE-6195 by Erik Hatcher (@erikhatcher), updated May 09 2016

[asfimport]

Erik Hatcher (@erikhatcher) (migrated from JIRA)

I'm interested in using Markdown/pegdown with some other Solr files too - e.g. solr/example/files/README.txt. Assigning to myself to keep it on my visible TODO list

[asfimport]

Uwe Schindler (@uschindler) (migrated from JIRA)

Hey, the infrastructure is there. I am sure we can include more *.md or *.txt files to the pegdown processing. The ANT task is already prepared, so this fould be just extending the fileset and placing the files at correct location!

[asfimport]

Erik Hatcher (@erikhatcher) (migrated from JIRA)

unassigning to 5.3, won't get to this by then