Update dependencies (Gatsby v2 to v5)

[sophialittlejohn]

This PR:

• updates all dependencies (migration from Gatsby v2 to v5 and all plugins)
• adds eslint and fixes linting errors
• adds syntax highlighting to md files (will indicate missformated text, tables and expressions)
• adds prettier and fixes formatting
• adds linting and prettier checks to build command to enforce code style and formatting

Open tasks

• [x] fix redirect behaviors
• [x] fix katex/math plugin integration
• [x] check image loading
• [x] table rendering

Notes:

• Comments in markdown have to be written as js and not as html: {/* ... */}
• Native html characters need to be escaped in markdown:
  • ex 1: <> --> {'\<>'}
  • ex 2: Polkadot{js} --> `Polkadot{.js}`
• Some math expressions are way too long and take up more than 100% of the page width, I'd suggest refactoring or introducing variables to make the rendered version shorter
• when adding a new dependency the gatsby develop server often errors out, if it happens use the new yarn nuke script which will clear the cache and reinstall all dependencies
• docs uses yarn v4, beware when installing and building

[gpmayorga]

CI issues solved!