Enable doc generation for source-build

[carlossanlop]

Fixesdotnet/source-build#2877

This is a forward port of this 6.0 change: #59937

Will backport to 7.0.

Kudos to @MichaelSimons for helping me find the missing change.

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Issue Details

---

Fixes dotnet/source-build#2877

This is a forward port of this 6.0 change: #59937

Will backport to 7.0.

Kudos to @MichaelSimons for helping me find the missing change.

Author:	carlossanlop
Assignees:	carlossanlop
Labels:	area-Infrastructure-libraries
Milestone:	-

[carlossanlop]

/backport to release/7.0-rc2

[github-actions]

Started backporting to release/7.0-rc2: https://github.com/dotnet/runtime/actions/runs/3100059329

[carlossanlop]

I asked, and it is not needed for RC2, so I'll backport it to GA directly.

[carlossanlop]

/backport to release/7.0

[github-actions]

Started backporting to release/7.0: https://github.com/dotnet/runtime/actions/runs/3100139684

[carlossanlop]

CI is all green. The llvmfullaot leg shouldn't be testing anything from this change.