docs: Reshape Server fundamentals + add Configuration reference

[developerjamiu]

Phase 2 IA, PR 1 of 7 (tracked in #671): reshapes the Server fundamentals pages into concept pages and stands up a Configuration reference.

What changed

• Running your server (00-start-command): reshaped from a flag reference into a concept page about the development loop, hot reload, migrations from the terminal, and run modes. The exhaustive flag tables now live in the CLI reference, which the page links to.
• Configuration: split into a concept page (how configuration works, the config files, Dart config, secrets, and code generation) and a new Configuration reference holding the exhaustive option tables.
• Sessions and Modules: light conformance (page titles, descriptions, cross-links). Modules now points at Configuration for the generator.yaml module options instead of repeating them.

Correctness fixes

• Removed the --flutter-device and --flutter-option flags (they do not exist in the CLI) and corrected the --flutter description, in both the page and the CLI reference.
• Removed the inheritance experimental feature from the docs; it is a stable feature now (the experimental feature set is all only).
• Updated the github, google, and facebook auth-provider links after the Configuration headings were reorganized.

Verification

The Configuration reference tables were checked row-by-row against the framework config source (serverpod_shared/lib/src/config.dart and environment_variables.dart): all environment-variable names, config-file keys, and defaults match. Three keys that were missing have been added with verified defaults: database.dialect (postgres), database.dataPath (embedded Postgres), and validateHeaders (true). healthCheckInterval and experimentalDiagnosticHandlerTimeout are Serverpod() constructor arguments, not config-file options, so they are intentionally not in the table.

Notes

• Structural changes (grouping pages into sidebar categories, the section rename, redirects) are intentionally deferred to the final PR of this series, so this PR keeps every page at its current URL.

Closes #664.

[Zfinix]

LGTM 🚀 Approved but a rebase is needed