LaTeX macros for individual package documentation

[mahrud]

It would be great if each package could define its own macros for documentation. Currently these are the default: https://github.com/Macaulay2/M2/blob/11396850f9d3bbe84aae00dfdb0f77093e93c89e/M2/Macaulay2/m2/html.m2#L22-L29