Update internal developer docs to use new configuration guidance

[domoscargin]

Fixes #5501

Considerations

There is considerable overlap between these docs and the new docs on the website. I've mostly taken the approach of favouring the website, cutting out content and instead linking out to relevant bits of the site guidance, but the guidance here was really nice, and there are some bits that now feel a bit orphaned (ie: everything to do with Nunjucks).

I'd favour moving all the topics from this doc to the site and having a single documentation source.

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-development.min.css	118.41 KiB
dist/govuk-frontend-development.min.js	42.78 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	92.92 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	87.29 KiB
packages/govuk-frontend/dist/govuk/all.mjs	1.18 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	1.74 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend.min.css	118.4 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	42.77 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.55 KiB
packages/govuk-frontend/dist/govuk/init.mjs	7.13 KiB

Modules

File	Size (bundled)	Size (minified)
all.mjs	82.76 KiB	40.37 KiB
accordion.mjs	26.62 KiB	13.41 KiB
button.mjs	9.13 KiB	3.78 KiB
character-count.mjs	25.43 KiB	10.9 KiB
checkboxes.mjs	7.81 KiB	3.42 KiB
error-summary.mjs	11.03 KiB	4.54 KiB
exit-this-page.mjs	20.24 KiB	10.34 KiB
header.mjs	6.46 KiB	3.22 KiB
notification-banner.mjs	9.39 KiB	3.7 KiB
password-input.mjs	18.28 KiB	8.33 KiB
radios.mjs	6.81 KiB	2.98 KiB
service-navigation.mjs	6.44 KiB	3.26 KiB
skip-link.mjs	6.4 KiB	2.76 KiB
tabs.mjs	12.04 KiB	6.67 KiB

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