Document package sources for self update

[Mirochill]

Pull Request Check List

Resolves: #10633

• Added tests for changed code.
• Updated documentation for changed code.

Summary:

• Document that poetry self commands use a system pyproject.toml in Poetry's configuration directory.
• Clarify that poetry self update reads package sources from that system project, not from the current project's pyproject.toml.
• Link the [[tool.poetry.source]] format to the package sources documentation.

Validation:

• git diff --check
• rg -n "system project|configuration#config-directory|repositories/#package-sources|tool\.poetry\.source|current project.*self update|preserves existing" docs\cli.md

Not run locally: documentation build, tests, or pre-commit.

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