This PR:

1. Adds markdown spellchecking and linting to tox and Github Actions.
2. Fixes existing linting problems and spelling in docs/**/*.md & README.md.

Motivation

We now have CD of our docs from the main branch and we should try and maintain a certainly level of quality:
https://ethereum.github.io/execution-spec-tests/main

External Tooling Required

• spelling: Uses the pyspelling Python cli tool which requires the external aspell command. This is a standard package and can be installed on Debian-based distros via:

  sudo apt-get install aspell aspell-en
  

• lint: Uses the markdownlint-cli2 command-line utility. This is node.js program, so introduces quite a hefty dependency. If you don't already use node.js (and require multiple versions), then you can install markdownlint with:

  sudo apt install nodejs
  sudo npm install markdownlint-cli2 --global
  

  Experienced users would probably use nvm to manage and install different node versions, see https://www.freecodecamp.org/news/node-version-manager-nvm-install-guide/.

Tooling and Integration with VS Code:

• Markdown spellchecking is already covered in VS Code via the Code Spell Checker that we already recommend in our extensions.json.

• Dictionaries: Now "whitelist.txt" is split into 3 separate wordlist files for maintainability. One reason: If you do "Quick Fix" -> "Add word to dictionary 'project-words'", VS Code sorts the wordlist alphabetically, which is not so optimal for our list of opcodes. Pyspelling supports multiple wordlists. flake8-spellcheck works only supports one wordlist file, which must be called whitelist.txt. This is generated using the src/entry_points/create_whitelist.py in tox prior to running flake8.

• Btw, I did check whether we could spellcheck the markdown via the flake8-spellcheck plugin for simplicity, but it was not possible.

• I used the vscode-markdownlint for a couple of weeks - it works well. This extension uses the markdownlint-cli2 and there's a corresponding Github Action that uses this tool.

If you use these two plugins and clean-up any errors, then tox should pass.

Soft Fail Mode Rational

It's not ideal to require external (non-native Python) tooling and these tools are only required for people who work heavily on the docs. As such, if these tools are not available, tox prints a hint that they're missing, but will still pass. Most devs will only be working on the tests and should not break the docs or need to change them (docstrings in tests will get spell-checked from flake8; linting the output is more difficult, perhaps there's a flake8 plugin extension?).

Keeping tox cross-platform (hopefully)

Additionally, the tools are executed via Python scripts in src/entry_points to avoid using external commands (e.g. bash) in tox to ensure it still runs cross-platform.

Github Actions

These checks will always be ran in Github actions:

• Spellcheck is ran as part of tox (it's easy to install the aspell dependencies).
• Lint is ran using DavidAnson/markdownlint-cli2-action@v11 to avoid dealing with node.js dependencies in the workflow.