grass.tools: Add Tools API to generated tools doc

[wenzeslaus]

This adds grass.tools.Tools from #2923 to the generated documentation of tools as a new tab similar to the grass.script tab. The documentation and treatment of parameters is the same at this points in grass.tools as in grass.script except for io.StringIO.

The io.StringIO type is added based on the gisprompt on a best-guess basis. The current parser metadata don't contain explicit machine-readable information about stdin being allowed. Parser then does not know whether or not a specific tool supports stdin for one of its parameters. The stdin is resolved while opening the file by a library function call at the level of individual tools, but during command line parsing, the parser merely allows for dash being provided instead of a filename. The resulting documentation now contains "'-' for standard input" or the like based on what is in each tool. This is slightly misleading for the Tools API because "-" won't do anything useful because the only way to supply stdin at this point is io.StringIO. However, without extending the parser metadata, I don't see any way how to make it better.

The example creates a Tools object called tools and then calls its method:

tools = Tools()
tools.r_info(map="name", format="json")

Example with r.basins.fill:

The documentation contains text:

This API is experimental in version 8.5 and expected to be stable in version 8.6.

Comparison with the grass.script tab for r.info which does not have many parameters:

grass.script (only)	grass.tools

The io.StringIO piece in r.in.ascii documentation:

[wenzeslaus]

This is now waiting for #2923 to be merged. The CI failure (addons) is unrelated.