Updated custom dialect doc page so links work correctly in GitHub UI

[sfyz]

Summary

Made updates to custom dialect page so the links work correctly from the GitHub UI (and not just when hosting the doc site manually).

• Renames dialects/agentforce/agent-script.md → dialects/agentforce/index.md as the Agentforce category landing page
• Updates sidebars.ts to use dialects/agentforce/index for both the category link: and as the first child item in the TOC
• Updates migrate-sfdocs skill to write the intro page as index.md and emit the correct sidebar ID on re-migration

Test plan

• Verify "Dialects > Agentforce" category link in sidebar navigates to the intro page
• Verify intro page appears as the first item when the Agentforce category is expanded
• Run pnpm start in apps/docs and confirm no broken links

[sfyz]

Updated a documentation page so the links work correctly when browsing from github.com (previously only worked when hosting the site).... FYI @elijahbenizzy @sberryman