[DOC] Add developer guide for pytorch-forecasting (v1 + v2)

[echo-xiao]

LLM-assisted, human-directed content, produced with Claude Opus 4.8.

Reference Issues/PRs

Closes #2301 (part of the v2 documentation umbrella #2304).

What does this implement/fix? Explain your changes.

Adds docs/source/developer_guide.rst, a contributor-facing developer guide
covering both the v1 (stable) and v2 (active redesign) APIs, wired into the
Tutorials toctree.

Structure: Overview → Setup / CI → Architecture (v1 / v2 / v1→v2, with two
mermaid diagrams and a v1↔v2 comparison table) → Adding a contribution (shared
conventions, the official extension templates for models / data modules /
metrics, and migrating a v1 model to v2) → Verifying & submitting → Roadmap →
Getting help.

Design choices:

• Delegates the fork/clone/editable-venv setup and PR best-practices to the
  existing installation.rst rather than duplicating them.
• Delegates deep architecture reference to the existing v2 pages (data_v2,
  models_v2, pkg_v2, api_v2) and links the v2 tutorial notebooks for
  runnable end-to-end examples.
• Emphasises v2 and the v1→v2 developer migration, since that is the active
  work, and distinguishes it from the user-facing code migration guide.

What should a reviewer concentrate their feedback on?

• This PR adds one documentation build dependency, sphinxcontrib-mermaid (to
  the [docs] extra in pyproject.toml and to conf.py), used only to render
  the two architecture diagrams. I'm happy to drop the diagrams and the
  dependency for a zero-dependency page if you prefer — the ReadTheDocs PR
  preview shows them rendering.
• Whether the placement (under the Tutorials toctree) and the v1/v2 balance
  read well.

Did you add any tests for the change?

No — documentation-only change. The docs build cleanly (make html) with no new
warnings; all cross-references resolve and the ReadTheDocs PR preview renders
correctly.

Any other comments?

Content is grounded in the codebase — every class, method, tag, and CI job
referenced was verified against the source (extension templates, base classes,
.github/workflows/test.yml, pyproject.toml).

PR checklist

• The PR title starts with [DOC].
• Added/modified tests — N/A (documentation only).
• Used pre-commit hooks.

[echo-xiao]

hi, @phoeenniixx this developer guide (#2301) is the general contribution guide, and the model migration guide (#2288, pr #2349) is the detailed v1→v2 model-migration companion. once both are merged, a small follow-up pr will add the bidirectional :doc: cross-links between them.

[codecov]

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[phoeenniixx]

Nice start!
I have a few comments

• There is a lot of use of em dashes (--) we should avoid that, makes it awkward, no?
• I can see explaination of v1 and v2, you can simply point to v1 and v2 API for this, no need to explain in the developer guide.
• There is a table for v1 to v2 - should that be in the migration guide instead?
• FOr the extension-templates, it would be great if we could add the link to the templates as well
• Also, for the CI, I am not sure we need to explain the matrix - the devs care more about the test framework just? @fkiraly what do you think?

[phoeenniixx]

I think we should add it to getting-started and installation page, no? We need to make it easily accessible. Rn it is too deep in the stack

[phoeenniixx]

hi, @phoeenniixx this developer guide (#2301) is the general contribution guide, and the model migration guide (#2288, pr #2349) is the detailed v1→v2 model-migration companion. once both are merged, a small follow-up pr will add the bidirectional :doc: cross-links between them.

Suggestion:
You can stack these PRs: https://www.sktime.net/docs/developer-guide/git-workflow/#working-on-a-chain-of-dependent-tasks-pull-request-stacking

[echo-xiao]

hi, @phoeenniixx thx for the review. i have already stacked the two PRs, so the cross-links are in place. the diff includes #2349 until it merges, then i'll rebase to clean up.

• remoted dashes
• trimmed the v1/v2 architecture parts
• removed v1->v2 table, now points to migration guide
• dropped the ci matrix, i think there is not need to keep it, but i can always restore it if @fkiraly prefer it
• movedthis to the top-level nav

for pr #2349 , the migration guide is now linked from the beta box at the top of every v2 page.