Neoteroi-mkdocs

- Adds the possibility to specify a `class` for the root HTML element of `cards`.
- Fixes a bug in the `contribs` plugin (adds a carriage return before the
contribution fragment).
- Modifies the packages to group all extensions under `neoteroi.mkdocs` namespace.
- Replaces `setup.py` with `pyproject.toml`.

- Corrects the pattern handling name and email for the `contribs` plugin.
- Adds tests for the `contribs` plugin.

- Corrects bug in the `contribs` plugin, causing failures in certain CI/CD
solutions and improves its safety and performance (removes `shell=True`!).
- Adds option to apply a class to specific contributors, for simpler styling.
- Minor improvements to styles.

- Improves the `contributors` plugin:
- - Adds the possibility to exclude contributor information (for example to not display
contributions from bots)
- - Adds the possibility to merge contributors' information
for cases in which the same person commits using two different email addresses
- - Adds the possibility to display an element with the count of contributors
- Resolves a small issue in pip package resolution

- Improves the Gantt extension:
- - now supports multiple periods in the same row
- - now supports activities using the start date from the previous activity (automatic dates)
- Adds a contributors plugin (`neoteroi.contribs`) to display contributors'
information in each page, obtaining information from the Git repository at
build time :star:

- Adds common classes to enable custom extensions reading configuration from:
- - YAML, JSON, or CSV embedded in the markdown source
- - YAML, JSON, or CSV fetched from a URL at build time (simple HTTP GET), with the
option to define a custom fetcher for example for use cases that require
authentication
- Adds the `Cards` extension, to render cards
- Adds the `Timeline` extension, to render timelines
- Adds the `Projects` extension, with a control to render `Gantt` diagrams :sparkles:
- Adds the link to the [documentation site](https://www.neoteroi.dev/mkdocs-plugins/)
- Drops support for Python 3.7 in the build pipeline (code might still work but is not ensured)