⚙️ [Maintenance]: Remove instruction files colocated into the docs site (#349)

The Markdown and PowerShell style instruction files have been removed
from this repository. Their content now lives in the PSModule
documentation site under the new **Style Guides** section, giving
contributors and agents a single, browsable source of truth for coding
conventions.

## Changed: Instruction files moved to the docs site

The `.github/instructions/md.instructions.md` and
`.github/instructions/pwsh.instructions.md` files (and the now-empty
`instructions` folder) have been removed. The same guidance is published
— and expanded with a new GitHub Actions guide — at the docs site:

- Companion PR: PSModule/docs#43 adds the Style Guides section.

This is an internal maintenance change. It does not affect the shipped
artifact or any workflow behavior.

## Technical Details

- Deleted `.github/instructions/md.instructions.md` and
`.github/instructions/pwsh.instructions.md`.
- Content migrated verbatim to `src/docs/Style-Guides/Markdown.md` and
`src/docs/Style-Guides/PowerShell.md` in `PSModule/docs`.

## Downstream coverage

The MSX documentation site consumes this guidance and adds the few
authoring conventions that were not carried over verbatim — image alt
text, PowerShell matching operators (`-like`/`-match`/`-c`) and
read-only constants, and requirement-identifier formatting — in
[MSXOrg/docs#16](MSXOrg/docs#16).

Author: MariusStorhaug