Fix incorrect f-string format

There was a small error that did not occur with Python 3.12 but only with earlier versions, so I had missed it.
See #107 for more details and #106 for the fix.

Added the new `servers` configuration

Now a pool of servers can be used for remote rendering.
Servers are tried in order, the first responding is used.
The old server and kroki_server configurations are now deprecated.
See the README.md for more details.

Fixed search in include directories

Fixed a small bug when finding files to include if there is more than one directory specified in base_dir.

Handled Mkdocs path directives in the configuraton file

Now directives like !relative $docs_dir are handled correctly in the base_dir plantuml_markdown configuration.
Now you can set search paths relatives to MkDocs project path.

Fixed source search in `base_dir` paths

This version fixes a small but annoying bug that prevented proper inclusion of sources in the presence of multiple paths configured via base_dir.

Fixed inclusion of plantuml config file with local rendering

Last version introduced a bug which caused errors when trying to locate the PlantUML config file when used with local plantuml command.

Fixed include from multiple paths

Now the base_dir configuration accept also a list of strings, interpreted as a list of paths where to search for files to include.

Fixed namespace in inline SVG

This version fixes a small bug with inline SVG, which added an ns0: before every SVG tag.
There is also a JsonSchema file to use with MkDocs-Material to validate the configuration options for the plantuml_markdown plugin.

Fixed json support

Fixed PlantUML json support when the json is defined in an included file or variable.

Auto-create cachedir if missing

Another minor improvement/fix: the path indicated in 'cachedir' is now automatically created if it doesn't exist.