chore: enable build-specific documentation roots

[david-christiansen]

This PR enables the use of the build-time configuration of the Lean reference manual URL and updates the release checklist to account for the reference manual.

This is a follow-up to #7240, after the required stage0 update.

The release process described here uses the same location for the reference manual for RCs and stable releases. This is for two reasons:

1. The only changes between them should be a modification of the embedded version string and updates to the final release's release notes, once those are included.
2. It ensures that a compatible manual is available at the moment that the new release appears, so any delay getting it deployed won't be visible to users.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase d1d2f215adc90e88120f6b4cdda8d264c0e5de00 --onto 8fc8e8ed19ef218022f5a94cbf5e472e3b777e44. You can force Mathlib CI using the force-mathlib-ci label. (2025-03-12 13:30:10)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase d1d2f215adc90e88120f6b4cdda8d264c0e5de00 --onto 3d0f41e32321147b19c5a26651e3f0a3c6c0ab6a. You can force Mathlib CI using the force-mathlib-ci label. (2025-03-21 23:17:57)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase d1d2f215adc90e88120f6b4cdda8d264c0e5de00 --onto 12a21e79c71880424321d62e60449863c504048a. You can force Mathlib CI using the force-mathlib-ci label. (2025-03-31 08:29:17)

[david-christiansen]

@kim-em I think I've got this down to a pretty efficient procedure. I've tested the CMake update locally, and it did what it should.

Thanks for your patience while I got it all set up!

[jcommelin]

LGTM