doc: Clean up and improve documentation

[finger563]

Description

The docs had grown organically into a flat doc/en/ directory where ~23
category folders (imu/, display/, network/, …) coexisted with ~53
individual component pages dumped at the top level, and the master index.rst
interleaved both in one ~80-entry toctree. This reorganizes the documentation
around a consistent, discoverable structure without changing any component
content.

Navigation — the master index.rst is now grouped into captioned sections:
Introduction · Core & RTOS · Storage & Configuration · Buses & I/O · Sensors ·
Motor Control · Displays, LEDs & Haptics · Input & HID · Connectivity &
Protocols · Math & DSP · Data & Serialization · Dev Boards (BSPs).

Structure changes:

• Moved the ~53 flat component pages (and their _example.md companions) into
  category folders via git mv (history preserved): new core/, storage/,
  buses/, leds/, motor_control/, data/, protocols/; folded input
  devices into input/ and color into math/.
• New dev_boards/ section collecting all 19 board-support packages, grouped
  by vendor (Espressif, Adafruit, LilyGo, M5Stack, MotorGo, Seeed, Waveshare,
  Other) — previously these were scattered through the flat top-level list.
• Added a Getting Started page (getting_started.rst) surfacing the
  install/usage info that previously only lived in the repo README.
• Added per-section index.rst landing pages and cross-links between related
  areas (LEDs↔led_driver, Motor Control↔bldc↔filters, MotorGo boards↔motor
  control, Math↔filters, Protocols↔network).

Component manifests:

• Updated the documentation: URL in 57 idf_component.yml files to the new doc
  paths (e.g. spi.html → buses/spi.html, button.html → input/button.html,
  board pages → dev_boards/<vendor>/…), including the shared logger page
  anchors used by the logger, format, and binary-log components.

Motivation and Context

A reader scanning the old top-level index couldn't tell what the categories were
or where to find a board vs. a driver vs. a utility, and 16+ dev boards were
buried among system components. Grouping by capability (plus a dedicated Dev
Boards section and a Getting Started landing page) makes the docs navigable and
gives every page a consistent home. No API or example content changed — only
where pages live and how they're linked.

How has this been tested?

• Programmatically verified the new toctree structure: all 287 doc pages are
  reachable from index.rst, with no orphaned pages and no broken toctree
  references, and all RST title underlines are valid.
• Verified moves don't break references: esp_docs include-build-file:: inc/…
  resolves from a fixed build root (not file-relative), \snippet uses Doxygen
  example IDs, and each .rst moved together with its _example.md so
  document-relative toctrees stay valid.
• Verified all 116 active documentation: links across the component manifests
  resolve to real pages on disk (116 valid, 0 broken).
• Built the documentation locally (./build_docs.sh) and visually reviewed the
  rendered navigation.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.
• My code follows the code style of this project.

[github-actions]

✅Static analysis result - no issues found! ✅