README: add Docs Lookup for DevDocs to Related Projects

[sserpolar]

Adds one row to the Related Projects table for a Chrome/Edge extension I published this week.

What it does: select an identifier on any page, right-click, and the DevDocs entry
opens in a small top-level window beside your work. The original tab is never touched.

• Repo: https://github.com/sserpolar/devdocs-lookup (MIT)
• Chrome Web Store: https://chromewebstore.google.com/detail/docs-lookup-for-devdocs/ffkgfiafblighmadmkplbelldbjocjnk
• Edge Add-ons: https://microsoftedge.microsoft.com/addons/detail/ijociolldkbchmfofaelcbnaocoghggj

How it differs from the existing entries: the other browser-side entries are editor or
launcher integrations. This one acts on a selection on a third-party page — which is what
#1518 concluded devdocs itself cannot do:

Changing the context menu requires a browser extension … thus, is out of scope of devdocs.

Notes for review:

• It opens devdocs.io in an ordinary top-level window. Nothing is embedded, injected or
  modified. An earlier iframe-overlay version was abandoned precisely because DevDocs stores
  preferences in cookies without a SameSite attribute (treated as Lax), which a cross-site
  iframe cannot read — a top-level window keeps devdocs.io first-party, so the user's own
  enabled documentation sets and offline data apply.
• One permission (contextMenus), no host permissions — it cannot read page content.
• The store listing and README both state it is unofficial and not affiliated with
  DevDocs or freeCodeCamp.
• Row inserted in description alphabetical order (after "Brackets extension", before
  "Electron application"), per the <!-- table is sorted by description --> comment.