feat: Lazy build with --skip-initial-build

[lawndoc]

It would be awesome if --skip-initial-build would watch for requests and build requested paths as they are requested.

Lazy builds would massively speed up site serve time for large sites.

I'm willing to contribute to get this feature added, but I'll need some initial direction on where that would need to be implemented.

[jekyllbot]

This issue has been automatically marked as stale because it has not been commented on for at least two months.

The resources of the Jekyll team are limited, and so we are asking for your help.

If this is a bug and you can still reproduce this error on the latest 3.x-stable or master/main branch, please reply with all of the information you have about it in order to keep the issue open.

If this is a feature request, please consider building it first as a plugin. Jekyll 3 introduced hooks which provide convenient access points throughout the Jekyll build pipeline whereby most needs can be fulfilled. If this is something that cannot be built as a plugin, then please provide more information about why in order to keep this issue open.

This issue will automatically be closed in two months if no further activity occurs. Thank you for all your contributions.

[UlyssesZh]

Really need this! I am currently working on migrating the math engine on my blog from client-side MathJax rendering to server-side KaTeX rendering. However, it is pretty slow to render all the math in my blog at once, and site building becomes pretty painful for me. Event with incremental building, the first build is still painfully slow.