Require BifurcationKit 0.8 in docs

[ChrisRackauckas-Claude]

This PR should be ignored until reviewed by @ChrisRackauckas.

Summary

Follow-up to merged #4746. Restrict the documentation environment to BifurcationKit 0.8, the only BifurcationKit release line supported by current ModelingToolkitBase that also provides the Collocation API now used by the periodic-orbit tutorial.

This changes documentation-only compatibility metadata. It does not change ModelingToolkit's runtime dependencies or public API.

Compatibility boundary

• BifurcationKit commit 542afa4e0d948ee6f74575d9d5d43d2e7fc1e6ec renamed PeriodicOrbitOCollProblem to Collocation; the first containing release is v0.7.0.
• Current ModelingToolkitBase supports BifurcationKit 0.4, 0.5, 0.8.
• Therefore 0.8 is the only supported release line that can execute the tutorial merged in Fix documentation examples for current dependency APIs #4746. The previous docs compat 0.5, 0.6, 0.7, 0.8 advertised versions that cannot all provide that API in the current package stack.

Local verification

• Fresh docs resolution on the Fix documentation examples for current dependency APIs #4746 tree with this compat line selected BifurcationKit 0.8.0 and the local ModelingToolkit/ModelingToolkitBase packages: exit 0.
• Exact periodic-orbit tutorial path through continuation(..., Collocation(20, 5)) and get_periodic_orbit: exit 0, 101 orbit points.
• Full Julia 1.10 documentation build with all examples, strict external link checks, and HTML rendering: exit 0. Documenter's documented GITHUB_TOKEN link-check hook was used after two unauthenticated runs completed all examples but GitHub throttled its own NEWS.md link with HTTP 429; no link was ignored and no check was downgraded.
• Runic 1.7.0 --check --verbose over every tracked Julia file: exit 0, 219/219 files.
• git diff --check upstream/master...HEAD: exit 0.

[ChrisRackauckas-Claude]

Implementation scratchpad / commit record for d668d203e2466130626143a4f89d064d158bdb80:

1. Audited the API boundary: Collocation starts at BifurcationKit v0.7.0, while the current ModelingToolkitBase compat set is 0.4, 0.5, 0.8.
2. Reduced the docs compat declaration to the exact viable line, BifurcationKit = "0.8".
3. Verified a fresh docs resolve selects BifurcationKit 0.8.0 and ran the exact periodic-orbit continuation path successfully.
4. Ran the complete docs build on Julia 1.10.11. Two unauthenticated strict runs completed every example and then reproduced GitHub HTTP 429 on the repository's own NEWS.md link. The final run used Documenter's built-in GITHUB_TOKEN request header, kept link checking enabled, and exited 0 after examples, link checks, and HTML rendering.
5. Ran Runic 1.7.0 over every tracked Julia file (219/219) and git diff --check upstream/master...HEAD; both exited 0.
6. Fetched both remotes before branching, before commit, and before the normal push; the branch was not behind upstream/master and no pre-existing remote branch was overwritten.

No runtime source, test configuration, warning policy, or link-ignore list changed.

[ChrisRackauckas-Claude]

CI follow-up: the documentation build completed successfully, so all four checks are green. This matches the local Julia 1.10 strict docs build, which executed every example and external-link check successfully with no Documenter warn-only/configuration weakening.