Add docs for if-diff

[zanete]

Why: In relation to #637
What: Add documentation if-diff feature that's implemented in #637

Scope of work:

• feature is documented in a README in the if repository
• feature is documented on if.greensoftware.foundation in a dedicated page at https://if.greensoftware.foundation/reference/cli/if-diff (handled in add docs for if-diff if-docs#74)

[zanete]

@jmcook1186 Please check the documentation issue here, if this is what it's expected as part o the scope

[jmcook1186]

blocked pending merge of if-diff feature