* fix(core/gateway): option to limit directory size listing

* feat(gw): HTMLDirListingLimit

This is alternative take on the way we limit the HTML listing output.
Instead of a hard cut-off, we list up to HTMLDirListingLimit.
When a directory has more items than HTMLDirListingLimit we show
additional header and footer informing user that only $HTMLDirListingLimit
items are listed. This is a better UX.

* fix: 0 disables Gateway.HTMLDirListingLimit

* refactor: Gateway.FastDirIndexThreshold

see explainer in docs/config.md

* refactor: prealoc slices

* docs: Gateway.FastDirIndexThreshold

* refactor: core/corehttp/gateway_handler.go

#8853 (comment)

* docs: apply suggestions from code review

Co-authored-by: Alan Shaw <alan.shaw@protocol.ai>

Co-authored-by: Marcin Rataj <lidel@lidel.org>
Co-authored-by: Alan Shaw <alan.shaw@protocol.ai>