Add Philosophy/Vision page to docs site

[EtanHey]

The docs site is missing a Philosophy page that explains BrainLayer's vision:

BrainLayer as the primary knowledge source for AI agents — replacing scattered CLAUDE.md files with queryable memory.

Scope:

• Create docs/philosophy.md
• Add to mkdocs.yml nav
• Cover: why local-first, why MCP-native, the vision of memory-as-primary-knowledge-source

Getting started:

• Look at docs/index.md and docs/architecture.md for writing style
• The README intro section has good framing to build on