explodepkg, pkgdiff, pkgtool, upgradepkg: add pages

[osmium1]

Adds documentation pages for Slackware package management utilities.

Checklist

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR contains at most 5 new pages.
• The PR is authored by me, or has been human-reviewed if it was created with AI or machine translation software.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known): Slackware 15.0
• Reference issue: Let's document: pkgtools #11874, Reach parity with cheat cheatsheets #3689

[Managor]

makepkg will have to become a disambiguation page https://github.com/tldr-pages/tldr/blob/main/contributing-guides/style-guide.md#disambiguations

Do that in a separate PR

[osmium1]

Noted, I'll handle the makepkg disambiguation in a separate PR.

[Managor]

Nono, the pkgdiff page can stay. We just need to find the best available documentation. If it is the groff link, then so be it.

[osmium1]

Okay, re-added pkgdiff with the groff man page URL for now.