RFR: Section on backporting [v6]

[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 incrementally with one additional commit since the last revision:

  Update src/index.md
  
  Co-authored-by: Andrew Haley <aph-open at littlepinkcloud.com>

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

Changes:
  - all: https://git.openjdk.java.net/guide/pull/66/files
  - new: https://git.openjdk.java.net/guide/pull/66/files/b107bbd3..ab420f92

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=guide&pr=66&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=guide&pr=66&range=04-05

  Stats: 1 line in 1 file changed: 0 ins; 0 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