forge(doc): group free functions, constants and errors in a single md file with the solidity file name

[Alexangelj]

Component

Forge

Describe the feature you would like

All free functions (including custom errors) are exported with their own markdown file, leading to a lot of singular pages with only one piece of code. Since they are not defined within an interface or contract but they are within a solidity file, there should be a singular markdown file exported for each solidity file which includes all of its free functions, constants and custom errors.

Free constants are already exported to a dedicated "constants" file with the solidity file name, so this should just be extended to include all free functions and variables.

Additional context

No response