docs: publish a project page on GitHub Pages

[Chemaclass]

Summary

Publishes a project page at https://php-lightning.github.io/lnaddress/ and fixes two
stale statements in the architecture doc.

Landing page

docs/index.html — one self-contained file (no CDN, no build step, light and dark
theme), aimed at someone who has never heard of a Lightning Address:

• What it is — the address is just HTTPS: bob@example.com resolves to
  /.well-known/lnurlp/bob
• Why self-host — the address lives on your domain, payments settle straight to your
  node, no new stack to operate, several identities per deployment
• How it works — the four steps of the LNURL-pay flow with the actual request and
  response payloads
• Quick start — install, both config files, the well-known route
• Under the hood — the Gacela module flow and how to use the facade without HTTP
• FAQ — custody, backends other than LNbits, multi-user, browser wallets, what is not
  implemented

.github/workflows/pages.yml deploys docs/ with upload-pages-artifact /
deploy-pages on pushes that touch docs/, and can be triggered manually.
docs/.nojekyll keeps the markdown guides from being run through Jekyll; the page links
to them on GitHub.

Doc corrections

While checking every claim in docs/ against the code:

• EmptyBackendInvoice was described as the null object used for users without a backend.
  InvoiceFactory::getBackendForUser() always builds LnbitsBackendInvoice, so the class
  is currently unused — the table now says so.
• The "not implemented" section mentioned an image-metadata TODO that no longer exists
  after the LnurlPayMetadata extraction. Only the LUD-12 commentAllowed TODO remains.

Everything else in the README and docs/ was verified against the current source:
config setters and their defaults, response keys (pr, status, reason), routes,
composer scripts, and the CI job list.

Notes

GitHub Pages must have its source set to GitHub Actions for the workflow to publish.

Test plan

• composer test-all green (26 tests / 33 assertions)
• HTML parses with no unclosed tags; all links checked
• Pages deployment succeeds after merge