docs: add supported-remotes.md — clarify remote compatibility for OSS users

[AkshatRaj00]

Summary

This PR adds a new documentation page website/docs/introduction/supported-remotes.md that clearly explains which remote repositories are supported by Sapling for open-source users.

This directly addresses the documentation gap described in issue #1287.

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Problem

New Sapling users — especially those coming from Git — are frequently confused about the remote support story. The current docs don't have a dedicated page that answers: "Can I use Sapling with GitLab? With a self-hosted Gitea? What happened to Mercurial support?"

This confusion is a real adoption barrier. Users spend time experimenting or asking on issues (e.g., #1287, #1186) to find out what should be a one-minute read in the docs.

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What This PR Adds

• A new page supported-remotes.md in the Introduction section of the docs
• A quick-reference table showing support level for: Git remotes, GitHub, GitLab, Bitbucket, Gitea, Mercurial remotes, Mononoke, and a Sapling-native server
• Clear explanation of .git mode vs native sl mode with copy-paste examples
• A dedicated GitHub integration section covering sl pr, ISL, and sl ghstack
• A "Why no Mercurial or Mononoke?" section to preempt FAQ questions
• A link to issue proposal(pr): use github api directly rather than CLI for github operations #1148 for users interested in contributing non-GitHub review integrations

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Why This Matters

"Right now, the practical impression is that Sapling is mainly a better client for Git repositories, but that is not stated as clearly as it probably should be." — #1287

This page makes that statement explicit and complete, so potential adopters immediately understand the supported workflow and don't have to piece it together from scattered issues and installation notes.

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Checklist

• Docs only — no code changes
• New file added to existing introduction/ section
• All remote types covered with accurate support status
• Links to related issues included
• Consistent with existing docs tone and Docusaurus markdown format

Closes #1287

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