Workflow for updating pinned version

[thomas-zahner]

#113 introduced a reference to the lychee repository because it generates the Command-Line Flags page from source. With this change we also decided to pin the lychee version to keep the the docs reproducible.

Create a workflow step which either

• regularly (e.g. once a day) checks for newer versions and creates a PR to bump to the latest version.
• runs in the main lychee repository and creates a PR in this documentation repository to bump to the latest version.

The version is pinned in src/lychee-version.ts

[thomas-zahner]

Closed by #130