manual: add missing info for running separate parsers and renderers

[jenshalm]

This addresses the lack of crucial details in the manual that surfaced in this discussion: #489

Affected section: https://typelevel.org/Laika/latest/02-running-laika/02-library-api.html#separate-parsing-and-rendering

Changes:

• Mentions that the OperationConfig should be shared between parsers and renderers.
• Explains that theme configuration needs to be passed to both, parsers and renderers.

FYI @hmf