Added PDF watermark feature with customizable text, display, and position settings

[brij-mandaliya]

Fixes Issue : Feature Request: Watermarked Documents

closes #34

Changes proposed

• Add Watermark feature with customizable text, display and position
• Positions:
  • Top Left
  • Top Right
  • Bottom Left
  • Bottom Right
• Also add test case for watermark, Test will check file size with and without watermark difference

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• My change requires changes to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.
• This PR does not contain plagiarized content.
• The title of my pull request is a short description of the requested changes.

Screenshots

Note to reviewers