* doc/FAQ.org: Document for optimizing Spacemacs startup

[sunlin7]

Document for optimizing the Spacemacs by customize the variable load-suffixes and load-file-rep-suffixes.
On my local, Spacemacs/Emacs-30.1 startup time on Windows will be reduced from ~2.2 seconds to ~1.5 seconds.

353 packages loaded in 1.529s (e:266 r:19 l:5 b:63)

[fnussbaum]

Interesting, thanks! I'm a little bit hesitant to add this to our FAQ though, because I imagine users adding this to their dotfile, forgetting about it, and later installing some package that relies on dynamic modules. So I fear this might result in issues that are difficult to debug.

[bcc32]

I'm inclined to agree with the hesitation. Perhaps at least, we should include a strong warning that certain packages that Spacemacs uses do in fact use dynamic modules (vterm).

(I know that vterm is not enabled for Windows builds, but users may simply read the FAQ and not realize they are in a section that should only apply to Windows. I made the same mistake when reviewing this PR)

[sunlin7]

@fnussbaum @bcc32
Thanks for your comments, and sorry for late response, totally understand your concerns. And they're narrow so I just add them to FAQ.org to let other Spacemacs users know the options.
I change the example code snippets to give message to users who want try the options to understand the impacts.
Please help review again. Thank you.