RFR: Section on backporting [v8]

[Jesper Wilhelmsson]

> Guidance on backporting.
> Also moved "Backing out a change" to the testing section where I believe it fits better. This section is unchanged.

Jesper Wilhelmsson has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 10 commits:

 - Merge branch 'master' of github.com:openjdk/guide into backport
 - Fixes after review
 - Update src/index.md
   
   Co-authored-by: Andrew Haley <aph-open at littlepinkcloud.com>
 - Updated todo-list
 - Merge branch 'backport' of github.com:JesperIRL/guide into backport
 - Update src/index.md
   
   Co-authored-by: Magnus Ihse Bursie <mag at icus.se>
 - Update src/index.md
   
   Co-authored-by: Magnus Ihse Bursie <mag at icus.se>
 - jdk<release>u-fix-yes
 - Fixes after reviews
 - Section on backporting

-------------

Changes: https://git.openjdk.java.net/guide/pull/66/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=66&range=07
  Stats: 230 lines in 2 files changed: 135 ins; 94 del; 1 mod
  Patch: https://git.openjdk.java.net/guide/pull/66.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/66/head:pull/66

PR: https://git.openjdk.java.net/guide/pull/66