[OUDS] docs: add 'Getting started > Introduction' page

[louismaximepiton]

Related issues

Listed in #2589.

Description

This PR adds the "Getting started > Introduction" page based on:

• the previous corresponding Boosted page (see code source)
• the corresponding Bootstrap page (see code source)

⚠️ Needs to merge- ⚠️ :

• Does the type need to be added to the CSS to display the page ? IMO we can keep as-is (and add the codepen demo when the type is added).
• Do we keep the polyfill for focus-visible ? Or do we drop it ? It is quite related to our browserlistrc, do we change it or stick to the Bootstrap one ?
• BS Side: There is a Twitter link that should be changed to X atm

Types of change

• New documentation (non-breaking change which adds functionality)

Live previews

• https://deploy-preview-2658--boosted.netlify.app/docs/0.0/getting-started/introduction/

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