Issue : Inconsistent file extension for exporters #113
Description
The file_extension attribute in HTMLExporter, LatexExporter, and MarkdownExporter is set to 'txt'. Given that these exporters generate HTML, LaTeX, and Markdown files respectively, the file extension should reflect the output format. This inconsistency can lead to confusion and incorrect file handling by users. It would be more appropriate for the HTML exporter to have a .html file extension, .tex for latex and .md for markdown.