Add LinkedIn company profiles for author socials

[Hashim1999164]

Preflight checklist

• I have read the Contributing Guidelines on pull requests
• This is a code change: unit tests cover the new linkedin_company handle expansion.

Motivation

linkedin author socials always expand a handle to a people profile (/in/). Company pages live under /company/ instead, so a company handle currently 404s or hits the wrong person. Issue #12452 asked for a dedicated linkedin_company platform.

This keeps linkedin unchanged and adds linkedin_company next to it, with the same LinkedIn icon.

Test Plan

• authorsSocials unit tests: 9 passed
• Handle linkedin: sebastienlorber still becomes a people URL
• Handle linkedin_company: docusaurus becomes a company URL
• Full HTTP URLs still pass through unchanged
• Case insensitive keys still normalize

Test links

Deploy preview: once Netlify posts it, I will fill this in.

Related issues/PRs

Closes #12452

[netlify]

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[slorber]

Not really needed, see #12452 (comment)