docs(knowledge-graph): document the Knowledge Graph end to end

[mangglesh]

Summary

The Knowledge Graph had one page, which covered only the canvas basics and was out of date in places:

• It gave a 2,000-node limit; the real limit is 1,500.
• It described animated, labelled edges; they are plain arrows.
• It put the legend behind an info icon; it is the Relationships button.

It said nothing about where the data comes from, how fresh it is, the full set of node and relationship types, or where NudgeBee uses the graph. This PR turns the page into a section, Features → Semantic Knowledge Graph, at the same URL:

• Overview: what the graph is, where to open it, what is in it, how fresh it is, and who can see it.
• Explore the Graph: every filter, search and focus, clicking through, Path and Back/Forward, the canvas controls, the 1,500-node limit and the empty states.
• Nodes and Relationships: reading a node card, node and edge details, type versus sub-type, and reference tables for every node and relationship type the graph produces.
• Where the Data Comes From: each inventory and flow source and what it needs, links between sources, freshness and removal timing, and the Coverage settings.
• How NudgeBee Uses the Graph: NuBi's dependency answers, alert impact and grouping, blast radius on recommendations, and service criticality.

The section opens by default. Nested sidebar sections now use the same case as the pages beside them, and only top-level groups stay in capitals; this applies across the site.

Everything here describes what is shipped on main today. Manual Declarations (#867) will stack on top of this PR.

Type of change

• New documentation page
• Update to existing documentation

Checklist

• I ran npm run build locally and the site builds without errors
• I ran npm run lint locally and lint passes
• Internal links work; new external links open the correct page
• My commits are signed off (DCO — git commit -s)
• I read CONTRIBUTING.md

Additional notes

• Fact check: every claim was checked against the enterprise main source, about 230 claims in all, and the 31 errors that pass found are fixed.
• Left out on purpose: two flow sources that only an operator can enable through environment variables have no Helm setting, so they aren't documented.
• Screenshots: the existing screenshots and walkthrough GIF are still current and are reused. Focus mode and the Path bar have no screenshots yet.

[gemini-code-assist]

Code Review

This pull request restructures the 'Semantic Knowledge Graph' documentation by moving it from a single flat file into a dedicated directory with multiple sub-pages (Overview, Exploring, Nodes and Relationships, Data Sources, and How NudgeBee Uses the Graph) and a category configuration. It also updates all referencing links across the documentation and adds custom CSS to style nested sidebar items. The reviewer feedback consistently identifies spelling inconsistencies, specifically pointing out several instances of British English spellings (such as 'Organisations', 'Centre', 'neighbours', 'summarises', 'neighbourhood', 'organised', 'organisation', and 'greyed') that should be updated to US English to maintain consistency throughout the documentation.