docs: fix broken links across the documentation site (#865)

Checked all documentation links with lychee (the same checker used in
apify-docs, run over the built site) and fixed the broken ones. Doc-link
404s drop from 379 to 10.

- **Frozen versioned API reference**: source links were hardcoded to
`blob/master/…`, which started 404ing after the
`clients/resource_clients/`→`_resource_clients/` and
`client.py`→`_apify_client.py` refactor. Pinned them to the immutable
version tag (e.g. `blob/v2.5.1/…`), mirroring how apify-sdk-python pins
them to a commit SHA.
- **Model docstrings**: absolutized relative `](/platform/…)` links and
fixed the malformed Docker build-cache anchor. Added
`absolutize_doc_links()` to `scripts/postprocess_generated_models.py` so
regeneration keeps them absolute.
- **Changelog**: corrected release-tag URLs (`releases/tags/` →
`releases/tag/`, and `v1.3.0` → `1.3.0` since that tag has no `v`
prefix).
- Applied the matching docstring fixes to the committed version
snapshots (0.6/1.12/2.5/3.0) so the published pages are correct now.

The remaining 10 are theme/plugin artifacts (changelog "Edit this page"
links pointing at generated files, an llms-txt `blog.md` link, a frozen
1.12 cross-reference) plus a local-only `reference/next` Docker link
that regenerates correctly in CI.