Fixes the failing NPM/netlify build

[Ilyesbdlala]

On the server side of our website, v16.15.0 (npm v8.5.5) has been updated to node v16.15.1 (npm v8.11.0) . Log with older version: https://app.netlify.com/sites/docs-securecodebox/deploys/62977867bbce6900088a87eb Log with newer version: https://app.netlify.com/sites/docs-securecodebox/deploys/62a07cb2bc39e20008e4b91d The current docusaurus 2.0.0-beta.14 version does not work on node v16.15.1 (npm v8.11.0).

@rseedorff Maybe you can shine a light on how the versioning is handled on the netlify server?. If we're using the latest tag, this issue will probably happen again.

This PR :

• updates docusaurus to 2.0.0-beta.21 and the corresponding function calls.
• updates the README to reflect the actual node/npm versions that are used server-side.

When merged, this should allow the latest documentation added to actually appear on the website.

[netlify[bot]]

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