docs(signals): rewrite testing guide

[rainerhahnekamp]

This is a rewrite of the testing guide for the SignalStore

• Reorganize into 8 chapters: scopes, members, unprotected, mocking deps,
  signalMethod, rxMethod, mocking store, custom features
• use Vitest
• more concise examples
• remove integration tests, long async snippets
• Add writing-docs skill for consistent doc conventions

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #5106

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[rainerhahnekamp]

@markostanimirovic @timdeschryver and others.

This is a complete rewrite, so the review is easier if you don't check the diffs line by line 😄

[markostanimirovic]

Great work Rainer! 👑

[timdeschryver]

Looks good to me as well! 🫡

[rainerhahnekamp]

@timdeschryver Incorporated your suggestions