RFR: Section on sponsoring [v3]

[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.

Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:

  Fixed review comments

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Changes:
  - all: https://git.openjdk.org/guide/pull/97/files
  - new: https://git.openjdk.org/guide/pull/97/files/f8701085..7b3e38e7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=guide&pr=97&range=02
 - incr: https://webrevs.openjdk.org/?repo=guide&pr=97&range=01-02

  Stats: 5 lines in 2 files changed: 0 ins; 2 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