Document use of configuration environment variables

[jmooring]

This list should describe:

1. How to use environment variables instead of configuration settings (e.g., HUGO_DISABLELANGUAGES, HUGO_CACHEDIR etc.)
2. Environment variables that do not have a corresponding configuration setting:

• DART_SASS_BINARY
• HUGO_ENVIRONMENT
• HUGO_FILE_LOG_FORMAT
• HUGO_NUMWORKERMULTIPLIER
• HUGO_MEMORYLIMIT

3. Precedence: command line flags override environment variables which override configuration settings (e.g., --cacheDir, then HUGO_CACHEDIR, then cacheDir in site configuration).

Related to item 3, include (somewhere) a list of which config settings can be overridden with a CLI flag.

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Edit: Item 2 was addressed by #2420 when documenting HUGO_MEMORYLIMIT in #2434.

[bep]

Note that the

• first bullet includes every config setting.
• HUGO_ENV was added to support a common legacy pattern and should not be included in the docs.

[jmooring]

This will be addressed with #1888