Project fails to build due to gemspec constraints and outdated jekyll version

## Problem description

Cloning the repository and running `bundle install` fails or behaves unexpectedly because of:

- A `.gemspec` file that restricts `jekyll` to `< 4.3`, preventing installation with Ruby >= 3.4.
- The use of `gemspec` in the `Gemfile`, which defers all dependency logic to the `.gemspec`.

As a result, contributors face errors or have to manually edit the project structure to get it working locally.

## Proposed solution

- Remove the `.gemspec` file entirely, since the project is a static website and not intended for gem publishing.
- Remove the `gemspec` line from the `Gemfile`.
- Move all necessary dependencies directly into the `Gemfile` with clear version specifications.
- Use `~> 4.2` for `jekyll` to remain compatible with Ruby 3.4+ and allow non-breaking patch updates.

## Benefits

- Simplifies setup with `bundle install`.
- Ensures compatibility with recent Ruby and Jekyll versions.
- Avoids confusion for new contributors.
- Removes unnecessary coupling to a `.gemspec`.