Restructure WGSL shader docs: reflection, capabilities, vertex/fragment

[mvaligursky]

Splits the former WGSL Specifics page into focused, cohesive pages. This is a pure documentation reorganization — it does not describe any new capabilities, so it stays accurate against the currently released engine.

Changes:

• WGSL Reflection (renamed from wgsl-specifics) — now scoped to simplified-syntax resource reflection only: Uniforms, Texture Resources, Storage Buffers.
• WGSL Capabilities (new) — optional, device-gated features grouped together: Half-Precision Types and WGSL language extensions (device.supports* / CAPS_*).
• WGSL Vertex and Fragment Shaders (new) — the vertex/fragment-only constructs: Attributes, Varyings, Fragment Outputs.
• Updated inbound links (compute-shaders, custom-shaders) and the shaders sidebar order: wgsl-reflection → wgsl-capabilities → wgsl-vertex-fragment-shaders.
• Added a client redirect from the old /user-manual/graphics/shaders/wgsl-specifics/ URL to wgsl-reflection.
• Anchors #half-precision-types and #wgsl-language-extensions preserved on their new page.

Follow-ups (not in this PR):

• i18n: the Japanese pages are left as a stale superset (the renamed JA reflection page still carries the moved sections in Japanese, so links resolve and the build stays clean). Trimming the JA reflection page and adding JA copies of the new pages is a translation follow-up.
• A later docs change will extend the WGSL Reflection page to compute shaders and lead the Compute Shaders page with the simplified path, once the corresponding engine support ships.