RFR: 7265: Update Release Notes for 8.1.0

[Marcus Hirt]

This is still a draft. It still needs, at least, an update for the actual issue numbers once we feel we're fairly stable, an update screenshot from Jira and a removal of unused images.

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Commit messages:
 - Adding image for approximate issue tally and updated ignore file
 - Image GC
 - Updates to 8.1 release notes
 - Updated release notes
 - 7265: Adding first few drafts

Changes: https://git.openjdk.java.net/jmc/pull/285/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=285&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7265
  Stats: 226 lines in 25 files changed: 222 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jmc/pull/285.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/285/head:pull/285

PR: https://git.openjdk.java.net/jmc/pull/285