docs: IDE integration chapter — VS Code and JetBrains pages in the main menu

[shejnowicz]

Follow-up to the docs-coverage audit of the last 3 months of merged PRs. Supported IDEs should be visible the moment a user opens the docs — a top-level sidebar section, not scattered mentions.

Gaps found:

• JetBrains path is essentially undocumented: fix(cli): emit customizations.jetbrains when --ide jetbrains #77 (customizations.jetbrains emission), fix(cli): set overrideCommand:false so app-init.sh runs in JetBrains #78 (overrideCommand: false — security-relevant: without it Gateway skips app-init.sh and bypasses the sandbox boundary), feat(cli): install JetBrains stack plugins via customizations.jetbrains #79 (per-stack Marketplace plugin auto-install) have no docs presence; neither do the extra JetBrains capabilities (DAC_OVERRIDE/CHOWN/FOWNER) or the .idea mount toggle beyond a table row.
• VS Code path is scattered: a dedicated hardening page exists, but the flow itself (devcontainer open, customizations.vscode, per-stack extensions) lives in one-liners across reference/cli.md and initialization.md.

Plan: new top-level toctree section IDE integration with two mirrored pages — ide/vscode.md (consolidation + pointer to the hardening page) and ide/jetbrains.md (Gateway flow, customizations block, per-stack plugins incl. the Ultimate-gating caveats, overrideCommand:false warning, capabilities, .idea mount) — plus IDE links on the landing page.