RFR: Section on sponsoring

[Jesper Wilhelmsson]

Copied and updated the text from the old sponsoring page: https://openjdk.org/sponsor/
The purpose is to have the sponsor link on the OpenJDK page lead here instead.

Also updated the contributing section with some parts taken from the old contributing page: https://openjdk.org/contribute/
Again, the intention is to change the link on the OpenJDK page to lead here.

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Commit messages:
 - Section on sponsoring

Changes: https://git.openjdk.org/guide/pull/97/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=97&range=00
  Stats: 59 lines in 3 files changed: 56 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/guide/pull/97.diff
  Fetch: git fetch https://git.openjdk.org/guide pull/97/head:pull/97

PR: https://git.openjdk.org/guide/pull/97