docs: add installing guide with correct Open VSX and VSCodium guidance

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Opened by @roomote-v0 on behalf of David Duatis

Adds a new docs/getting-started/installing.mdx page with tabbed installation instructions for VS Code, VSCodium/Open VSX, and JetBrains. Key additions:

• VSCodium / Open VSX tab with steps for installing from the Open VSX Registry (search or manual VSIX), plus an info callout noting the panel looks different from VS Code Marketplace
• VSCodium troubleshooting section with a platform-specific table of product.json paths (Linux system/Flatpak, macOS, Windows), the correct Open VSX gallery URL, and a warning about VS Code Marketplace URLs not working
• Navigation update in docs.json to include the new page under IDE Extensions > Getting Started

References draft PR: dduatis-star/Roo-Code-Docs#1

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View task on Roo Code Cloud

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Rooviewer   See task

Re-review complete. The previously noted MDX rendering pitfalls are still present in the current diff.

• Move <Info> out from inside <Step> to avoid Mintlify <Steps> child rendering issues
• Avoid <username> in Windows path (MDX may treat as HTML tag); use {username} or escape

Previous reviews

• cdaae2a: Review #1

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}

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<Info> is nested inside a <Step> here; Mintlify's <Steps> component often expects only <Step> children (and the existing install page keeps <Info> outside <Steps>), so this may render incorrectly. Consider moving this callout below </Steps>.

_{Fix it with Roo Code or mention @roomote and request a fix.}

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The Windows path uses C:\Users\<username>..., but in MDX the <username> can be parsed as an HTML tag and disappear in the rendered output. Using {username} or escaping the angle brackets avoids that.

_{Fix it with Roo Code or mention @roomote and request a fix.}

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Pushed a follow-up commit with three fixes:

1. Removed duplicate nav entry -- docs.json had both ide-extensions/install and getting-started/installing in the Getting Started group. Removed the old one so only the new page appears.
2. Added YouTube embed -- The original install page includes a YouTube iframe; the new page was missing it.
3. Aligned punctuation style -- Removed trailing periods from step text to match the existing convention used in ide-extensions/install.mdx (Mintlify docs in this repo omit sentence-ending periods in step descriptions and info callouts).