Add description of setting-up pre-commit CI for repos outside a GitHub organisation

The problem

The documentation for setting up pre-commit CI is primarily targeted at situations where the repository is part of a GitHub organization. This might confuse users that have a repo outside of a GitHub organisation. Here is a quote from the respective section in the documentation:

On GitHub, we can make use of pre-commit CI to run our pre-commit jobs (effectively our linting). This is configured by our .pre-commit- config.yaml file. The only step required to get it to run is to ensure that pre-commit CI is configured so that it has access to this repository. To do this, go to installations in your organisation's GitHub organisation (something like https://github.com/organizations/organisation-name/settings/installations or, alternately, https://github.com/organizations/organisation-name -> Settings -> GitHub Apps).

However, it is also possible to integrate pre-commit CI into a repository outside of an organization by visiting the website https://pre-commit.ci/ (link is already provided in the docs) and selecting the corresponding repository, which will install the GitHub App in that repository.

Definition of "done"

It would be helpful to add this information (or modify a bit the description) in the documentation for users whose repository is not part of an organisation.