Rename entities across the docs

[novusnota]

For simplicity, clarity, and, in some regard, conformance to grammar.ohm, I propose renaming of many terms throughout the docs.

Changes:

• global static functions -> global functions
• internal functions -> storage functions
• persistent state variables -> storage variables
• constants: global and storage ones
• getter functions: off-chain functions / accessors (preserve the older "getter" name alongside the new one too)