RFR: Add Premain EA link, Premain JEP links, link to training run doc

[John R Rose]

Some adds to the index page:

 - premain JEP and EA links
 - link to Dan's doc about training runs

One thing that baffled me a bit is the `<br/>` elements in the markdown.  These seem to be either hand-inserted as an attempt to do line length control, or else they are artifacts of an old cut-and-paste. Do we really want them?  If so, can we put in a comment in the markdown that says how to maintain them, please?

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Commit messages:
 - Add Premain EA link, Premain JEP links, link to training run doc

Changes: https://git.openjdk.org/leyden-docs/pull/4/files
  Webrev: https://webrevs.openjdk.org/?repo=leyden-docs&pr=4&range=00
  Stats: 7 lines in 1 file changed: 6 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/leyden-docs/pull/4.diff
  Fetch: git fetch https://git.openjdk.org/leyden-docs.git pull/4/head:pull/4

PR: https://git.openjdk.org/leyden-docs/pull/4