Add conformalforecast to nixtlaverse build pipeline

[nasaul]

Clones Nixtla/conformalforecast into both the preview and production builds and registers it in merge_docs.py so its navigation folds into the ConformalForecast anchor.

⚠️ Depends on #52 and #53 — merge this LAST

Order	PR	Base	Depends on
1	#52 — preview anchor	docs-preview	—
2	#53 — production anchor	docs	—
3	this PR	main	#52 and #53

#52 and #53 are independent of each other and can merge in either order. This PR must come after both: merge_navigation looks the anchor up by name and returns early when it is absent, so merging this one first copies the pages in with nothing linking to them.

What changed

• preview.yml / production.yml: a Clone conformalforecast step (ref: docs-preview / docs), the copy into the docs tree, and the staging fix below.
• merge_docs.py: conformalforecast added to anchor_name_map and repos.
• .gitignore: ignores a local conformalforecast/ clone so it never gets committed here — the pipeline fetches it at build time.

Carrying the #47 / #48 lessons up front

Both fixes are included from the start rather than after a broken run. cp -R brings the checkout’s .git along, which makes git add stage a newly added project as a gitlink instead of its files — the nav goes live while every page 404s. I reproduced it against the real checkout:

without fix -> mode=160000  files_staged=1     # gitlink, content-less pointer
with fix    -> mode=100644  files_staged=28    # real files

One deviation from #48: rather than adding a second git rm --cached line, conformalforecast joins the existing one as -- synforecast conformalforecast. Same behavior, and it keeps that line from growing once per package forever.

Verification

Full preview pipeline simulated end to end — the real docs-preview tree with #52 applied, the real conformalforecast@docs-preview checkout, rm -rf .git, cp -R, then the real merge_docs.py:

• ConformalForecast anchor: 5 groups / 22 pages
• every nav entry resolves to a real .mdx on disk
• light.png, dark.png, favicon.svg present; no stray .git, no pull/ preview dirs
• no other anchor emptied or reordered
• both workflow files parse as valid YAML, 12 steps each

Production will fail until conformalforecast has a docs branch

Nixtla/conformalforecast has no docs branch yet — nothing publishes to it except a release or a build-docs.yaml workflow_dispatch with deploy_target=production. Until that runs, the production clone step here fails and takes the nixtlaverse production build with it. Preview is unblocked today.

Recommendation: create that branch before merging this PR.