docs: add OSS upgrade & crew-to-flow migration guide (#5744)

* docs: add OSS upgrade & crew-to-flow migration guide

* docs: add upgrading-crewai guide and installation note

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* docs: consolidate upgrade & migration guide into single page

Merge the broader root-level upgrade-crewai.mdx into the canonical
en/guides/migration/upgrading-crewai.mdx so there is one comprehensive
upgrade & migration page covering: project venv vs global CLI, why
crewai install alone won't bump versions, breaking changes, and the
Crew-to-Flow migration. Removes the orphaned root-level file (which
was not referenced in docs.json nav).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* docs: add pt-BR, ar, ko translations of upgrade/migration guide

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* docs: reduce upgrade guide scope to package upgrade + breaking changes only

* docs: soften intro tone — releases ship features, not breaking changes

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* fix: resolve CodeRabbit review comments

- Add space between Arabic conjunction and `uv.lock` code span (ar)
- Add explicit {#memory-embedder-config} anchors to localized headings
  so in-page links resolve correctly (ar, ko, pt-BR, en)

Co-authored-by: Lucas Gomide <lucaslg200@gmail.com>

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Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: Lucas Gomide <lucaslg200@gmail.com>
Co-authored-by: Greyson LaLonde <greyson.r.lalonde@gmail.com>