Add starter documentation for addCustomMenu - #521

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-node@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

'e' is defined but never used

'selectionSets' is assigned a value but never used

'e' is defined but never used

'e' is defined but never used

'e' is defined but never used

'e' is defined but never used

'phylotree' is assigned a value but never used

'fs' is assigned a value but never used

'_' is assigned a value but never used