refactor(core): internalize, simplify and optimize the SSG logic

[slorber]

Motivation

Our static site generation logic initially comes from an unmaintained Webpack plugin, that we forked:

• Original project: https://github.com/markdalgleish/static-site-generator-webpack-plugin
• Our 2022-2024: https://github.com/slorber/static-site-generator-webpack-plugin

It's time to re-internalize this code, remove some unused code, and remove friction to be able to more easily optimize it.

This:

• decouples the SSG pipeline from webpack
• refactor/clean all webpack configs and start/build scripts
• add a basic perf logging system (internal for now)
• optimize SSG pipeline
• reduce server bundle size, moving dependencies (eta, minifier) out of the server bundle

The build time perf wins are not huge, because SSG executes relatively fast compared to the bundling phase.

yarn clear && yarn install && yarn build:website --locale en && hyperfine --runs 5 'yarn build:website --locale en'

Before
  Time (mean ± σ):     39.741 s ±  1.187 s    [User: 44.686 s, System: 6.152 s]
  Range (min … max):   38.486 s … 41.364 s    10 runs


After
  Time (mean ± σ):     37.416 s ±  1.405 s    [User: 42.558 s, System: 6.299 s]
  Range (min … max):   36.015 s … 39.096 s    5 runs

Test Plan

CI + unit tests

Test links

https://deploy-preview-9798--docusaurus-2.netlify.app/

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 78	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 87	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 76	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 71	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 65	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 71	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 76	🟢 100	🟢 100	🟢 90	🟠 88	Report

[github-actions]

Size Change: 0 B

Total Size: 986 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	75.1 kB
website/build/assets/css/styles.********.css	114 kB
website/build/assets/js/main.********.js	759 kB
website/build/index.html	37.9 kB

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