fix: replace GitHub brand SVGs with CC0 Simple Icons and declare in LICENSE (#1113)

[alaahong]

Fixes #1113

Summary

The website frontend bundles two GitHub brand mark SVGs
(website/src/pages/team/github.svg and website/static/img/github_icon.svg)
derived from GitHub's brand mark, which is not under the Apache License and
was not declared in LICENSE/NOTICE.

Following option (a) from the issue, this PR replaces them with the
ASF-compatible GitHub mark from Simple Icons, which is dedicated to the
public domain under CC0 1.0 Universal (ASF Category-A compatible), and
declares the assets in LICENSE with the full license text.

Changes

• website/src/pages/team/github.svg — replace the iconfont GitHub glyph with
  the Simple Icons GitHub mark (fill="currentColor", matching the existing
  .team-link .github-icon { color: #fff } style); remove the misapplied ASF
  header and add the CC0 attribution.
• website/static/img/github_icon.svg — the path data was already the Simple
  Icons mark; remove the misapplied ASF header, add the CC0 attribution, and
  keep the existing fill="black" rendering unchanged.
• LICENSE — add a "Simple Icons" third-party entry with the full CC0 1.0
  Universal legal text.

Licensing

Simple Icons is licensed under CC0 1.0 Universal
(https://creativecommons.org/publicdomain/zero/1.0/). The icons are declared
in LICENSE with the exact license text, satisfying the IPMC follow-up.

Verification

• Both SVG files are well-formed XML.
• No third-party GitHub artwork remains under a misapplied Apache header.
• Changes are confined to website/ assets and LICENSE; CI ignores
  website/** and no release artifacts are affected.

[psxjoy]

I don't think we need put license info into LICENSE file.
Can we just make a new LiCENSE file in website folder and split it into two projects?