[docs] add example of converting code block meta strings to required matches

[tlawrie]

Description

For those of us transitioning from a previous module to rehype-pretty-code we may already have code meta which would break prior versions of documentation.

For example

• title= may be filename= or heading=
• highlighting lines may be lines[x,z] or #x,z instead of {x,z}

[tlawrie]

Similar to: #52

Albeit we want to change the line number encapturing as well

[tlawrie]

Turns out this can be done by

      filterMetaString: (str) => str.replace(/lines=\[([^]*)\]/g, '{$1}').replace(/filename=([^ ]*)/g, 'title="$1"'),

This should be added to the documentation as an example.