chore: add MIT and Apache 2.0 license texts

[gesposito]

Adds the dual-license texts the README already promises, ahead of taking the repo public.

Details

• LICENSE-MIT and LICENSE-APACHE at the repo root, matching the license files the @gathertown/webhook-object-* npm packages ship with, so the examples repo and the published packages state licensing identically.
• The README's License section now links both files.

Test Plan

• Docs-only change, CI covers the rest.

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Note

Low Risk
Documentation and legal text only; no runtime or application code changes.

Overview
Adds root-level LICENSE-MIT (Gather Presence, Inc.) and full LICENSE-APACHE so the repo matches the dual-license statement already in the README and aligns with licensing on the published @gathertown/webhook-object-* packages ahead of making the repo public.

The README License section now links to both files and notes at your option, instead of only naming MIT and Apache 2.0 in prose.

^{Reviewed by Cursor Bugbot for commit 91f3b3a. Bugbot is set up for automated code reviews on this repo. Configure here.}

[linear]

GCO-2138 Add license txt files

Apache2.0, MIT

Review in Linear

[yaustar]

Why do we have both?

[gesposito]

Why do we have both?

Defer to @bgreenier