[skara] RFR: Improve the help for reviewers

[Magnus Ihse Bursie]

Rename "Download" to "Reviewing". Add information on how to get the PR using Skara tools and diff files. Collapse all information by default to make it more compact.

An example on how this will look can be found here: https://github.com/magicus/openjdk-sandbox/issues/1

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Commit messages:
 - Improve the help for reviewers

Changes: https://git.openjdk.java.net/skara/pull/1094/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1094&range=00
  Stats: 29 lines in 1 file changed: 24 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/1094.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1094/head:pull/1094

PR: https://git.openjdk.java.net/skara/pull/1094