pbmtoascii: add page

[deadpyxel]

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

This should progress #10887

[tldr-bot]

The build for this PR failed with the following error(s):

pages/common/pbmtoascii.md:3: TLDR003 Descriptions should start with a capital letter

Please fix the error(s) and push again.

[deadpyxel]

Not really sure if I should have rebased to keep a cleaner history. If so please let me know

[sebastiaanspeck]

The commits will be squashed eventually so it doesn't matter that much

[gutjuri]

Thank you very much for your contribution. I have added some suggestions.

[kbdharun]

Thanks for your contribution. I have some suggestions for this page.

[deadpyxel]

This command in particular just accepts -1x2 and -2x4. Should we reflect that on the page?

[kbdharun]

This command in particular just accepts -1x2 and -2x4. Should we reflect that on the page?

Yep, let me add a quick suggestion for it.