RFR: Minor cleanups after the JBS re-write

[Jesper Wilhelmsson]

Using markdown tables instead of html tables.
Added a separate style for JBS values.
Minor language fixes to make it the same style as the rest of the guide.
Clarified some sentences.
Added the label jep-superseded to the dictionary.

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Commit messages:
 - Minor cleanups after the JBS re-write

Changes: https://git.openjdk.org/guide/pull/106/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=106&range=00
  Stats: 210 lines in 4 files changed: 20 ins; 88 del; 102 mod
  Patch: https://git.openjdk.org/guide/pull/106.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/106/head:pull/106

PR: https://git.openjdk.org/guide/pull/106