Clarify Vault API limitations working w/ hidden (dot) directories

Context

Spent quite some time today trying to figure out why I couldn't access anything inside .obsidian using the Vault API (namely, getFileByPath()/getAbstractFileByPath()) until I asked on Discord and figured the way to go was using DataAdapter (vault.adapter).

Issue

I tested every function in the Vault API that takes a path as a parameter and tried manipulating test files/folders inside the .obsidian path. Functions that create or copy a file/folder do what they're meant to do, but return null instead of the relevant object.

Proposal

Document this behavior in the reference for the relevant functions and in the main Vault docs, suggesting the use of DataAdapter instead of the Vault API for hidden files.

Proposed wording

Based on how it's written in the DataAdapter documentation

Work directly with files and folders inside a vault. If possible prefer using the Vault API over this.

I suggest we append to each relevant page's description the following phrase:

Prefer using DataAdapter when working with hidden files and folders, such as those inside .obsidian.

Start with the TypeScript API Vault reference and the DataAdapter documentation linked in the issue, then review the pages for the listed Vault functions. Document the observed behavior for hidden files and folders and add guidance to prefer DataAdapter, including the main Vault docs; done means the relevant references clearly cover these limitations.