Update CONTRIBUTING.md:
* A more friendly approach, hopefully.
* The note about receiving patches is moved to the DCO section.
* Some "Get started" links in a bullet list (inspired by OpenTofu's
contributing file).
* For questions, refer to GitHub Discussions and Discord instead of only
Discord.
* Minor edits and formatting.
Update issue templates:
* The issue template for questions is replaced by a link to Discussions
and to Matrix and Discord chats.
* Add a link to the valkey-doc repo.
* The crash report template is extended into a form, with separate
fields for the crash report and the additional info.
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Signed-off-by: Viktor Söderqvist <viktor.soderqvist@est.tech>
