RFR: Section on project maintenance

[Jesper Wilhelmsson]

* New section on project maintenance
* New infographics on push/defer during rampdown
* Moved JDK Release Process section to follow the Release Note section
* Minor fixes

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Commit messages:
 - Section on project maintenance

Changes: https://git.openjdk.org/guide/pull/92/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=92&range=00
  Stats: 81 lines in 7 files changed: 76 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/guide/pull/92.diff
  Fetch: git fetch https://git.openjdk.org/guide pull/92/head:pull/92

PR: https://git.openjdk.org/guide/pull/92