website: allow yarn start without Mercurial

[mangeshraut712]

Summary

yarn start for the public docs site crashed with Error: not found: hg because docusaurus-plugin-internaldocs-fb@1.19.1 calls which.sync('hg') without { nothrow: true }. The which package throws when the binary is missing, so hasHg() never returned false.

1.19.2+ already passes { nothrow: true }, so a missing hg is treated as "not in a Mercurial repo." CI already builds the site without Mercurial; this makes the public dev server match that.

Fixes #4612.

Changes

• Bump docusaurus-plugin-internaldocs-fb from ^1.19.1 to ~1.19.3 (resolved 1.19.3). ~ stays on 1.19.x and avoids 1.20.0's extra Docusaurus 2 mermaid dependency.
• Document the old failure mode in website/README.md. Public website development does not require Mercurial.

Testing

• Reproduced on a machine without hg: yarn start failed at hasHg() in 1.19.1.
• After the bump: yarn start --port 3001 printed Docusaurus website is running at: http://localhost:3001/ and compiled the client successfully.

AI usage

An AI coding assistant was used under my direction to reproduce the crash, identify the which.sync call in 1.19.1, and apply the version bump. I reviewed the plugin source and verified yarn start locally.

[meta-cla]

Hi @mangeshraut712!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!