Fix redundant puml processes

[ang-zeyu]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [x] Other, please explain: performance

Fully fixes #1243
Follow up to #1245 (comment)

What is the rationale for this request?
The puml plugin generated-diagram-tracking is ineffective as the data structure is cleared during preRender, which is called per page.

Let's add a per site (re)generation plugin hook to solve this

What changes did you make? (Give an overview)

• add beforeSiteGenerate hook
• implement this for the puml plugin, properly tracking the generated puml diagrams

Testing instructions:
The number of jvms spawned should only be the total number of puml definitions

Proposed commit message: (wrap lines at 72 characters)
Fix redundant puml jvm processes

The puml plugin tracks generated puml images, and does not regenerate
them if the same image has already been generated.

However, the Set data structure that tracks this is cleared during each
call of the plugin preRender hook, which is called per page.
Multiple pages may reference the same diagram, making the tracking
ineffective.

Let's add a new plugin hook, beforeSiteGenerate, that is called during
initial site generation and site regeneration during live preview.
This allows plugins to maintain and clear state during site
(re)generation.

Let's implement this hook in the puml plugin, and clear the Set data
structure in it instead, which properly tracks the generated puml
images.