[rust-analyzer] automatically populate init_options from settings["rust-analyzer"]

[emersonford]

rust-analyzer expects init_options to be set to the contents of settings["rust-analyzer"] per:

• https://github.com/rust-lang/rust-analyzer/blob/eb5da56d839ae0a9e9f50774fa3eb78eb0964550/docs/dev/lsp-extensions.md?plain=1#L26
• https://github.com/rust-lang/rust-analyzer/blob/eb5da56d839ae0a9e9f50774fa3eb78eb0964550/editors/code/src/ctx.ts#L213

This change ensures the Neovim config for RA complies with the docs, and eliminates a source of confusion around whether to define RA settings in settings or init_options.

[github-actions]

Note that server_configurations.md or server_configurations.txt will be regenerated by the docgen CI process. Edit the Lua source file instead. For details on generating documentation, see: https://github.com/neovim/nvim-lspconfig/blob/master/CONTRIBUTING.md#generating-docs