packaging: move documentation HTML building to own function
This is part of some light refactoring to enable us to use
PyOxidizer for WiX MSI installer generation.
Differential Revision: https://phab.mercurial-scm.org/D10685
BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2