Features to add to developer docs

[jesserockz]

Overview/High-level

• Basic introduction/home page
• Introduction to hardware interfaces (I2C, SPI, UART, GPIOs)
• Various commonly-used consts/variables (CONF_*, AUTO_LOAD, to_code, etc.)
• FAQ

Code

• Basic component CONSTs (AUTO_LOAD, DEPENDENCIES, CODEOWNERS, MULTI_CONF, MULTI_CONF_NO_DEFAULT)
• Always use the api generation script if you modify api.proto
• use walrus operator for optional config gathering - sensor_config := config.get(CONF_SENSOR) - except for boolean values.
• No AUTO_LOADing the main platform components (sensor, binary_sensor etc)
• Any literal string used more than once should be defined as a constant.
• Configuration keys (i.e. those that appear as keys in YAML) should be defined (even if only used once) as constants of the form CONF_XXX where XXX is the upper-case version of the YAML key.
• Configuration constants used only in a single component should be defined within that component. If the same constant is used in 2 or more components it may be migrated to esphome/const.py. If it appears in 4 or more components it must be migrated to esphome/const.py (will fail CI checks otherwise.)
• Coroutine priorities (see [core] Update some coroutine priorities esphome#6755)

Tests

• All components should have test yaml files located in tests/components/<component name>/test.<platform>.yaml and this yaml should contain every configuration option being set and any actions and conditions the component has.

Documentation

• Documentation example is to be minimum required to compile, optional configs not to be included. Pin values should always be the string REPLACEME
• Example should not include dependent components, but a sentence and link to the relevant configuration documentation
• If a component is used exclusively, or primarily, on a single specific board (i.e. with dedicated pin numbers) a complete configuration for the component on that board may be included as a sample at the end of the doc, clearly identified as being for that hardware. This should not replace the generic example configuration.

Traps for the unwary

• If you just added a new validation function and your code blows up, check that the validation function has return value at the end.