feat (format/graph): add `graphfieldspages` param to optionally include Page-type values as node fields

[gesinn-it-ilm]

Currently, the graphfields=yes option displays only non-Page (e.g., string, date) property values in the graph node info table. Page-type is excluded and rendered only as edges.

To improve flexibility, we propose introducing a new parameter:

|graphfieldspages=no

Behavior:

• Default: no (to retain current behavior)
• When set to yes, Page-type properties are also included as fields in the graph node info table (alongside being used for edges).