Add dedicated docs page for init-module

[awaelchli]

What does this PR do?

Fixes #18083
Fixes #18216

Adds a standalone documentation guide for init_module and its use cases.
Covered for both Fabric and Trainer.
Linked in pages where appropriate.

Before submitting

• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing, make sure you have read the review guidelines. In short, see the following bullet-list:

Reviewer checklist

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

cc @Borda @carmocca @justusschock @awaelchli

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 fabric: Docs

Check ID	Status
docs-checks (fabric, doctest)	success	✅
make-html (fabric)	success	✅

These checks are required after the changes to docs/source-fabric/advanced/model_init.rst, docs/source-fabric/advanced/model_parallel/fsdp.rst, docs/source-fabric/api/fabric_methods.rst, docs/source-fabric/examples/index.rst, docs/source-fabric/fundamentals/precision.rst, docs/source-fabric/glossary/index.rst, docs/source-fabric/guide/checkpoint.rst, docs/source-fabric/guide/index.rst.

🟢 pytorch_lightning: Docs

Check ID	Status
docs-checks (pytorch, doctest)	success	✅
make-html (pytorch)	success	✅

These checks are required after the changes to docs/source-pytorch/advanced/model_init.rst, docs/source-pytorch/advanced/model_parallel/deepspeed.rst, docs/source-pytorch/common/index.rst, docs/source-pytorch/common/precision_basic.rst, docs/source-pytorch/common/precision_intermediate.rst.

---

Thank you for your contribution! 💜

Note
This comment is automatically generated and updates for 60 minutes every 180 seconds. If you have any other questions, contact carmocca for help.