Rewrite the README, fix the docs badge and the dark-mode live_info table

[basnijholt]

Description

A pass over the README and a few small docs fixes.

README:

• Rewritten in plain prose, closer to the original README: no bold-labelled feature list, the learner list as before, and the installation and development steps kept. The "Key features" section is gone, and the docs home page no longer includes it.
• Added a self-contained script example with BlockingRunner next to the notebook example, and a line on to_numpy()/to_dataframe().
• Filled the gaps the old feature list left: the summary says adaptive is meant for functions of a few parameters, the learner section explains the loss function (with the built-in alternatives and the custom loss tutorial), and the executor paragraph says the runners use all local cores by default and how to pass another executor.
• Plain pip install adaptive as the install line, with the [notebook] extra explained; the development section has the test command and a correct note on the notebook git filter.
• Badges: the Read the Docs badge did not load as an image (readthedocs.org redirects to app.readthedocs.org), so it now uses img.shields.io/readthedocs/adaptive. Added a tests badge for the nox workflow and dropped the GitHub stars badge.
• Removed the doctoc table of contents (GitHub shows an outline) and the TOC generator workflow that maintained it.

Docs:

• In dark mode, the live_info table had dark rows with black text inside the white widget card, because the theme gives tables a dark background. The card now uses the light theme colors.
• The footer no longer says "Made with Zensical" (extra.generator = false).
• The parallelism tutorial no longer claims that Windows defaults to distributed.Client; loky is the default everywhere.

Both README examples were run: the script with BlockingRunner, and the notebook one with live_info/live_plot.

Checklist

• Fixed style issues using pre-commit run on the changed files
• pytest passed (docs and CSS only; uv run --group docs pytest docs/ -o addopts="" passes, and zensical build --strict reports no issues)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• (Code) style fix or documentation update
• This change requires a documentation update