`yamllint.linter.run("example.yaml", yaml_config)` example seems
`yamllint.linter.run` opens a given file.
It's misleading.

rstcheck succeeds with a failure (heh) when there's a code block without
a language specified. This can lead to false negatives as the file is no
longer being checked by rstcheck.

Error:

    An `AttributeError` error occured. This is most propably due to a
    code block directive (code/code-block/sourcecode) without a
    specified language. This may result in a false negative for source:
    'docs/disable_with_comments.rst'. See
    https://rstcheck-core.readthedocs.io/en/latest/faq/#code-blocks-without-language-sphinx
    for more information.  Success! No issues detected.

As reported in #548, there
might be a problem with	pathspec 0.11.1 which does't allow calling
`match_file()` with argument `None` anymore.

The `linter.run()` function shouldn't call
`YamlLintConfig.is_file_ignored(None)` anyway.

Update syntax of pre-commit hook docs to work with newer pre-commit versions.

Closes #551, #553

According to the YAML specification [^1]:

- > It is an error for an alias node to use an anchor that does not
  > previously occur in the document.

  The `forbid-undeclared-aliases` option checks that aliases do have a
  matching anchor declared previously in the document. Since this is
  required by the YAML spec, this option is enabled by default.

- > The alias refers to the most recent preceding node having the same
  > anchor.

  This means that having a same anchor repeated in a document is
  allowed. However users could want to avoid this, so the new option
  `forbid-duplicated-anchors` allows that. It's disabled by default.

- > It is not an error to specify an anchor that is not used by any
  > alias node.

  This means that it's OK to declare anchors but don't have any alias
  referencing them. However users could want to avoid this, so a new
  option (e.g. `forbid-unused-anchors`) could be implemented in the
  future. See #537.

Fixes #395
Closes #420

[^1]: https://yaml.org/spec/1.2.2/#71-alias-nodes