feat(sd): added readme for dependency among SD host related components

components/esp_driver_sdmmc/README.md

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+# SDMMC Host Driver
+
+SD Host side related components are:
+- `sdmmc`
+- `esp_driver_sdmmc` (current component)
+- `esp_driver_sdspi`
+
+For relationship and dependency among these components, see [SD Host Side Related Component Architecture](../sdmmc/README.md).
+
+`esp_driver_sdmmc` components holds SDMMC Host driver for ESP SDMMC peripheral, this driver provides APIs to help you:
+- do SD transactions (under SD mode) via ESP SDMMC peripheral.
+- tune ESP SDMMC hardware configurations, such as clock frequency, bus width, etc.
+- ...
+
+You can
+- use this driver to implement `sdmmc` protocol interfaces
+- directly use `esp_driver_sdmmc` APIs
+
+to communicate with SD slave devices under SD mode.

components/esp_driver_sdspi/README.md

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+# SDSPI Host Driver
+
+SD Host side related components are:
+- `sdmmc`
+- `esp_driver_sdmmc`
+- `esp_driver_sdspi` (current component)
+
+For relationship and dependency among these components, see [SD Host Side Related Component Architecture](../sdmmc/README.md).
+
+`esp_driver_sdspi` components is a driver based on ESP GPSPI master driver to help you:
+- do SD transactions (under SDSPI mode) via ESP GPSPI peripheral.
+- tune ESP GPSPI hardware configurations, such as clock frequency, bus width, etc.
+- ...
+
+You can
+- use this driver to implement `sdmmc` protocol interfaces
+- directly use `esp_driver_sdspi` APIs
+
+to communicate with SD slave devices under SDSPI mode.

components/sdmmc/README.md

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+# SDMMC Protocol Layer Driver
+
+## SD Host Side Related Component Architecture
+
+### Architecture Graph
+
+                            ┌─────────────────────────────────────────────┐
+                            │                                             │
+                            │  SD Protocol Layer Driver: components/sdmmc │
+                            │                                             │
+                            └──────────▲────────────────────▲─────────────┘
+                                       │                    │
+                                       │                    │
+                            Implements │                    │ Implements
+                                       │                    │
+    ┌──────────────────────────────────┴─────┐         ┌────┴───────────────────────────────────┐
+    │                                        │         │                                        │
+    │Host Driver: components/esp_driver_sdmmc│         │Host Driver: components/esp_driver_sdspi│
+    │                                        │         │                                        │
+    └────────────────────────────────────────┘         └────────────────────────────────────────┘
+
+### Components
+
+- `sdmmc`: SD protocol layer driver, it provides SD protocol related definitions and interfaces. With corresponding implementation drivers, `sdmmc` APIs can help you:
+  - send commands to slave devices
+  - send and receive data
+  - handle error conditions within the bus
+- `esp_driver_sdmmc`: SDMMC Host driver for ESP SDMMC hardware, it implements the `sdmmc` protocol interfaces.
+- `esp_driver_sdspi`: SDSPI Host driver for ESP GPSPI hardware, it implements the `sdmmc` protocol interfaces.
+
+### Dependency
+
+- `esp_driver_sdmmc` is in driver layer (G2), it relies on `sdmmc`
+- `esp_driver_sdspi` is in driver layer (G2), it relies on `sdmmc`
+- `sdmmc` does not and should not rely on `esp_driver_sdmmc` or `esp_driver_sdspi`. Though `sdmmc` is independent, it still stays in G2.