Add spell checking for markdown files

### Description

Currently spell checking is not available in markdown files and it is impossible to enable it:


### Use case/motivation

Markdown files contain a lot of text so spell checking can help people to avoid spell mistakes.

### Related issues

_No response_

### Are you willing to submit a pull request?

No

The issue concerns adding spell checking to Markdown files in Apache NetBeans. Start by locating the existing Markdown support and spell-checking entry points, then verify the feature against the Markdown text shown in the issue; done means spelling errors are detected in Markdown files and the behavior is covered by the relevant existing checks.

Written by the indexing model from the issue text.