fix: add mkdir step for build-html script

[TusharThakur04]

Summary
This PR adds mkdir -p out to the build-html script so the directory is created automatically before doc-kit runs.

• it makes dev convenient.

Reproduce

What kind of change does this PR introduce?
fix

Did you add tests for your changes?
no

Does this PR introduce a breaking change?
no

If relevant, what needs to be documented once your changes are merged or what have you already documented?

Use of AI
no

[avivkeller]

cc @ovflowd we should probably create the output directory in doc-kit, tbh. I think we can implement that as a part of recursive file creation, which is my TODO for this weekend.

Also, this isn't a cross-platform solution.

[ovflowd]

There are reasons of why doc-kit does not create the directory for you, as many other tools out there, as it is not its job to manage the filesystem for you.

At times you want the directory to be created under a different account, or different chmod, or you accidentally left a directory that doesn't exist anymore, this will err on that.

So Im fundamentally -1 with this change.