Docs: split variables into sections

[AlexSkrypnyk]

Currently variables.md is one huge auto-generated file with variables.

Consider splitting it into multiple files automatically based on a list of keywords:

variables-extractor.sh \
... \

--sections(
  ACQUIA="Description of the section"
)

then, given DREVOPS_DB_DOWNLOAD_ACQUIA_KEY, the ACQUIA substring would be matched and extracted into a new section