docs: Extract Model reference and reshape Models concept pages

[developerjamiu]

Phase 2 IA, PR 3a of 7 (tracked in #671): reshapes the Models subgroup and extracts a Model reference. This is the first half of the Data & the database work; the Database subgroup follows in PR 3b.

What changed

• Model reference (new): the master model-keyword table moves out of Working with models into a dedicated reference. All 31 keyword rows are preserved, and every link is rebased. This becomes the single source that PR 3b's Database tables page defers to.
• Working with models: the keyword table is replaced with a pointer to the Model reference; added a description. All explanatory content and the per-type default tables stay on the page.
• Vector & geography fields: the two twin pages are merged into one, preserving both intros, all field types, and the warnings. The old pages are deleted and inbound links updated (the type anchors are unchanged).
• Custom serialization: added frontmatter; fixed a stray import (relation_test_shared to my_project_shared), an incomplete code sample, and a fluff word.
• Inheritance & polymorphism and Backward compatibility: cross-linked to each other; conformance.
• Shared packages: refreshed the version pins to the current 4.0-beta line (Dart ^3.10.3, Serverpod 4.0.0-beta.0).

Notes

• Version values track the current dev line (4.0-beta on main); the Dart SDK ^3.10.3 matches the framework template.
• Structural changes (grouping pages into a sidebar category, the section rename, redirects) are deferred to the final PR of this series, so this PR keeps every page at its current URL.
• The serverpod generate references on these pages are accurate (the command still exists); their framing is modernized separately in Update references to 'serverpod generate' to reflect the changes with hot reload #538, a dedicated cross-cutting pass so every page stays consistent.