[jdk15u-dev] RFR: 8264823: Update building.html document for Git in jdk15u

[Yuri Nesterenko]

To generate this HTML version of .md original, we had to 
install locally Pandoc v. 2.3.1 using make/devkit/createPandocBundle.sh, 
configure with PANDOC=path-to-pandoc-executable flag 
remove building.html  
and make  update-build-docs 

New version has only necessary changes for Git transition, I think. See also a backport of JDK-8251549.

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Commit messages:
 - 8264823: Update building.html document for Git in jdk15u

Changes: https://git.openjdk.java.net/jdk15u-dev/pull/19/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk15u-dev&pr=19&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264823
  Stats: 39 lines in 1 file changed: 4 ins; 23 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk15u-dev/pull/19.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk15u-dev pull/19/head:pull/19

PR: https://git.openjdk.java.net/jdk15u-dev/pull/19