RFR: 8300792: Refactor examples in java.net.http to use @snippet

[Darragh Clarke]

Refactored instances of `@code` to instead use `@snippet`.

I also spent some time looking into using external snippets to reference the existing files `JavadocExamples` and `WebSocketExample` though ultimately thought it was better to keep the docs pretty much as they had been with only a few small changes to formatting.

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

Commit messages:
 - updated copyright year and some more formatting changes
 - cleanup
 - converting @code tags to @snippet

Changes: https://git.openjdk.org/jdk/pull/12692/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12692&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300792
  Stats: 96 lines in 4 files changed: 28 ins; 5 del; 63 mod
  Patch: https://git.openjdk.org/jdk/pull/12692.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12692/head:pull/12692

PR: https://git.openjdk.org/jdk/pull/12692