docs: add command quick start next to README installation

[zackees]

Context

The root README describes fbuild as PlatformIO-compatible, but immediately after pip install fbuild users still lack a scannable command guide showing what to run.

The README should provide this orientation as part of the quick-start flow, before sending users into project setup or the detailed CLI reference.

One syntax detail should be explicit: fbuild has no standalone clean subcommand. Cleaning uses fbuild build --clean or fbuild deploy --clean.

Proposal

Immediately after the pip install fbuild installation block, add a concise “Command Quick Start” section containing two tables.

PlatformIO-compatible workflows

fbuild	PlatformIO equivalent
fbuild build	pio run
fbuild build --clean	pio run --target clean, followed by pio run
fbuild deploy	pio run --target upload
fbuild deploy --clean	Clean, then pio run --target upload
fbuild monitor	pio device monitor
fbuild ci	pio ci

Explain that --platformio delegates build, deploy, and monitor to the installed PlatformIO CLI for A/B comparisons, while fbuild ci provides a compatible fbuild-native CI surface.

Commands unique to fbuild

Add a grouped table containing every remaining top-level command, with a one-line explanation:

• Build and device workflows: reset, test-emu, compile-many, bringup
• Analysis and developer tooling: symbols, bloat, clang-tidy, iwyu, clang-query, clangd-config, lib-select
• Dependencies and caches: sync, purge, cache, lnk
• Runtime and device introspection: daemon, show, device, serial, port
• Integrations: mcp

Keep detailed flags and nested subcommands in docs/reference/cli.md; link that reference directly after the quick-start tables.

Acceptance criteria

• A “Command Quick Start” section appears immediately after the README’s pip install fbuild block.
• Users see the command mapping before the project-creation walkthrough or other detailed documentation.
• The PlatformIO mapping covers build, clean build, upload/deploy, clean deploy, serial monitor, and CI.
• Every example uses valid fbuild syntax and does not imply that fbuild clean exists.
• The section explains where --platformio is supported and distinguishes passthrough mode from fbuild-native compatibility.
• A separate grouped table lists every fbuild-only top-level command present in the current CLI when the change lands.
• Each fbuild-only command has a concise, user-oriented description.
• Each command appears exactly once in either the compatibility table or the fbuild-only table.
• The section links to docs/reference/cli.md for detailed flags, examples, and nested subcommands.
• The inventory is checked against the current Clap command surface or fbuild --help, including recently added commands such as cache.
• The tables remain concise enough to function as quick-start guidance rather than duplicating the full CLI reference.

Related issues

• docs: restructure README into scalable documentation categories #523 — restructured the README and established docs/reference/cli.md as the detailed CLI reference.
• feat(cli): normalize compile-many under a PlatformIO-compatible fbuild ci command #242 — introduced fbuild ci as the PlatformIO-compatible counterpart to pio ci.