RFR: Section on cloning [v10]

[Jesper Wilhelmsson]

> Added a section on cloning from GitHub, and an example of building on Mac. Also fixed two minor bugs that caused problems with syntax highlighting in Atom.

Jesper Wilhelmsson has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:

 - Legacy Mercurial section
 - Merge branch 'master' of github.com:openjdk/guide into clone
 - need to merge
 - Personal clone
 - Introduced SSH earlier
 - Changed to https for upstream
 - More updates after review
 - Updates after review
 - git switch
 - Updates after review
 - ... and 2 more: https://git.openjdk.java.net/guide/compare/013dd34f...933c2976

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Changes: https://git.openjdk.java.net/guide/pull/59/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=59&range=09
  Stats: 504 lines in 1 file changed: 203 ins; 253 del; 48 mod
  Patch: https://git.openjdk.java.net/guide/pull/59.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/59/head:pull/59

PR: https://git.openjdk.java.net/guide/pull/59