Document and use correct permissions in the GitHub workflows (#258)

When the intention is to potentially create a new GitHub issue, the
workflows in question need to set the permissions correctly. Otherwise
the workflow runs may fail with:

	Error: Resource not accessible by integration

which is an admittedly _very_ cryptic way to say that the `GITHUB_TOKEN`
used in the GitHub workflow run lacks a particular scope to perform the
desired operation.

Note: The `GITHUB_TOKEN` used to have read/write permissions for all
scopes by default, but that changed to read-only as per
https://github.blog/changelog/2023-02-02-github-actions-updating-the-default-github_token-permissions-to-read-only;
That is the reason why this here change is necessary.

This fixes https://github.com/lycheeverse/lychee-action/issues/194

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>