Merge branch 'bugfix/fixes_from_github' into 'master'

fixes for github issues

This MR includes commits from several Github PRs, as well some fixes for issues reported on Github and on the forum.

- compile PHY-related code only when WiFi is enabled
- fix macOS setup instructions
- fix some of the Sphinx warnings
- update docs index, improve structure of the readme-
- wifi: fix typos, rename ESP_ERR_WIFI_NOT_START to ESP_ERR_WIFI_NOT_STARTED
- sdmmc: fix explanation of flash voltage regulator configuration efuses
- sdmmc: change idx num of example
- fix i2c_get_period name
- fix esp_log_level_set function name in docs

See merge request !420