docs: comprehensive documentation refresh — remove stale references, add missing content

Root documentation:
- README.md: add Datums description, update addon load order and SDK references,
  fix project structure tree, update issue reporting guidance
- CONTRIBUTING.md: update submodule table (remove ztools, add gears/datums/solver),
  fix QSS guidance (single canonical source, not three copies)
- docs/ARCHITECTURE.md: update bootstrap flow (5 addons, split deferred timers
  between Create core and Silo addon), update load order and source layout
- docs/COMPONENTS.md: add Datums and Solver sections, update Gears description,
  fix Silo origin registration reference
- docs/KNOWN_ISSUES.md: create missing file referenced by CLAUDE.md and CONTRIBUTING.md
- docs/INTEGRATION_PLAN.md: update layer 5 diagram, fix load order references,
  update Phase 6 install rules, fix Layer 2/3/5 descriptions
- docs/OVERVIEW.md: add datums submodule entry
- docs/UPSTREAM.md: update Phase 1 directory table and Phase 4 submodule list

mdBook documentation (docs/src/):
- SUMMARY.md: replace dead architecture/ links with existing reference pages,
  remove deleted silo-server files, add new silo-server pages
- introduction.md: rewrite — replace ztools with current addons (Silo, Gears,
  Datums, KCSDK), update version to v0.1.5/FreeCAD 1.2.0
- guide/getting-started.md: update first-run addon list
- guide/installation.md: update verification console output
- guide/workbenches.md: rewrite — replace ztools with Gears, Datums, Solver
- guide/building.md: update submodule table, fix error message guidance
- development/contributing.md: fix scope example and issue reporting
- development/repo-structure.md: rewrite — add SDK, datums, gears, solver,
  reference/ folder; update submodule and key files tables
- development/writing-an-addon.md: fix priority range table
- reference/create-module-bootstrap.md: rewrite — reflect addon_loader system,
  split deferred timers between Create core and Silo addon
- reference/datum-creator.md: update from ZTools to datums addon paths and naming
- reference/glossary.md: add KCSDK entry, update FreeCAD version, remove ztools
  entry, update repository URLs table

docs/src/development/contributing.md

@@ -37,7 +37,7 @@ Use [Conventional Commits](https://www.conventionalcommits.org/):
 | `art:` | Icons, theme, visual assets |
 
 Scope is optional but encouraged:
-- `feat(ztools): add datum point creation mode`
+- `feat(datums): add datum point creation mode`
 - `fix(gui): correct menu icon size on Wayland`
 - `chore: update silo submodule`
 
@@ -45,7 +45,7 @@ Scope is optional but encouraged:
 
 Report issues at the [issue tracker](https://git.kindred-systems.com/kindred/create/issues). When reporting:
 
-1. Note whether the issue involves Kindred Create additions (ztools, Silo, theme) or base FreeCAD
+1. Note whether the issue involves Kindred Create additions (Silo, Gears, Datums, theme) or base FreeCAD
 2. Include version info from **Help > About FreeCAD > Copy to clipboard**
 3. Provide reproduction steps and attach example files (FCStd as ZIP) if applicable