docs: repoint two dead relref anchors into repositories.md

[simpleqt]

The anchors repositories#adding-a-repository (docs/libraries.md:131) and repositories#using-a-private-repository (docs/pyproject.md:801) no longer match any heading — they were orphaned when repositories.md was rewritten in #10941 (verified via git log -S and the heading list). Repointed at the existing #package-source-constraint and #private-repository-example sections, which cover the referenced content.

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[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="docs/libraries.md" line_range="131" />
<code_context>

 In order to publish to a private repository, you will need to add it to your
-global list of repositories. See [Adding a repository]({{< relref "repositories#adding-a-repository" >}})
+global list of repositories. See [Package sources]({{< relref "repositories#package-source-constraint" >}})
 for more information.

</code_context>
<issue_to_address>
**issue:** The “Package sources” link lands on the “Package Source Constraint” subsection, which only explains restricting dependency resolution to a source; it does not explain adding a user-level publishing repository. Readers following the publishing instructions therefore still cannot find the required repository configuration.

**Triggers:** When a library author follows the link to configure a private publishing repository.

**Suggested fix:** Link to `repositories#publishable-repositories`, which documents configuring repositories used for publishing.

```suggestion
global list of repositories. See [Package sources]({{< relref "repositories#publishable-repositories" >}})
```
</issue_to_address>

Sourcery assessment

Approval pending. 1 finding to address first.

Blocking findings: docs/libraries.md:131

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[sourcery-ai]

issue: The “Package sources” link lands on the “Package Source Constraint” subsection, which only explains restricting dependency resolution to a source; it does not explain adding a user-level publishing repository. Readers following the publishing instructions therefore still cannot find the required repository configuration.

Triggers: When a library author follows the link to configure a private publishing repository.

Suggested fix: Link to repositories#publishable-repositories, which documents configuring repositories used for publishing.

Suggested change

	global list of repositories. See [Package sources]({{< relref "repositories#package-source-constraint" >}})
	global list of repositories. See [Package sources]({{< relref "repositories#publishable-repositories" >}})