docs: use stricter validation options (#6096)
## Summary
`mkdocs` supports [validation rules for
links](https://www.mkdocs.org/user-guide/configuration/#validation),
which can be tightened to report more issues than the default
configuration. I used the recommended "maximal strictness" configuration
from the documentation.
Adding the `anchors` rule helped spot 4 errors:
```console
WARNING - Doc file 'guides/install-python.md' contains a link '../concepts/python-versions.md#python-distributions', but the doc 'concepts/python-versions.md' does not contain an anchor '#python-distributions'.
WARNING - Doc file 'guides/install-python.md' contains a link '../concepts/python-versions.md#discovery-order', but the doc 'concepts/python-versions.md' does not contain an anchor '#discovery-order'.
WARNING - Doc file 'guides/projects.md' contains a link '../concepts/projects.md#lock-file', but the doc 'concepts/projects.md' does not contain an anchor '#lock-file'.
WARNING - Doc file 'pip/environments.md' contains a link '../concepts/python-versions.md#discovery-order', but the doc 'concepts/python-versions.md' does not contain an anchor '#discovery-order'.
```
## Test Plan
Local run of the documentation + `mkdocs build --strict`.
