Migrate from Jekyll to Docusaurus

[alexluckett]

Proposed change

• Migrates us from Jekyll to Docusaurus and uses the @defra/docusaurus-theme-govuk theme. Makes us consistent with interactive-map.
• Removes a lot of hacky code we had to do to parse markdown structure and fix up links - now Docusaurus handles it all.
• Updated screenshot to show the latest version of the engine with the rebrand.
• Removes any references to our legacy term "DXT"

Old site

New site

Type of change

• Bug fix
• New feature
• Breaking change
• Misc. (documentation, build updates, etc)

Checklist

• You have executed this code locally and it performs as expected.
• You have added tests to verify your code works.
• You have added code comments and JSDoc, where appropriate.
• There is no commented-out code.
• You have added developer docs in README.md and docs/* (where appropriate, e.g. new features).
• The tests are passing (npm run test).
• The linting checks are passing (npm run lint).
• The code has been formatted (npm run format).

[sonarqubecloud]

Quality Gate failed

Failed conditions
2 New Major Issues (required ≤ 0)

See analysis details on SonarQube Cloud

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