update toc to add index.md

[v-savila]

for bug 1132957

[v-savila]

@Techwriter40, I've added index.md per work request from Samantha Roberton (see task at https://mseng.visualstudio.com/CSI/_queries?id=1132957) and have updated toc.yml in the conceptual content folder so that the TOC will appear in the landing page (see the mockup in task) as well, however, I wanted to sync with you to confirm if we should add index.md or should it replace overview,md.

[opbldsb1]

✅ Validation status: passed

File	Status	Preview URL	Details
sharepoint/docs-conceptual/toc.yml	✅Succeeded	View (sharepoint-ps)

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.

[kenwith]

This looks good to me. I will sync with @Techwriter40 (Kirk) and if any issues we can alter it. Thanks!