blog: Add a note on how to convert the google doc to markdown

[surajssd]

People could collaborate on blogs on a shared google document. So, add instructions on how someone can convert the google doc to markdown in Writing a blog section of contributor's guide.

• Install Pandoc.
• Download the Google document in "Microsoft Word (.docx)" format.
• Then go to the folder for writing the blog:

export INPUT_FILE=""                        # This should be the file with .docx extension
export OUTPUT_FILE=""                       # This should be the file with .md extension
cd content/en/blog/2024

pandoc -t markdown_strict --extract-media . $INPUT_FILE -o $OUTPUT_FILE