gdm: add page

[KristopherLeads]

• 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 page(s) follow the style guide.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

[tldr-bot]

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

pages/common/gdm.md:
Error: Parse error on line 12:
...nager application:- Prevent GDM from b
---------------------^
Expecting 'BACKTICK', got 'DASH'

Please fix the error(s) and push again.

[KristopherLeads]

A series of commands for the GNOME Desktop Manager from 16796. These are structured somewhat annoyingly, so they're separated into separate PRs:

• gdm - 17066
• gdm-binary - 17067
• gdmsetup 17068
• gdm-stop 17069
• gdm-restart - 17070
• gdm-safe-restart - 17071

[dmmqz]

Suggested change

	- Prevent sanitizing `ENV` variables that start with `LD_`:
	- Prevent sanitizing environment variables that start with `LD_`:

[dmmqz]

Suggested change

	- Show `gdm` version:
	`gdm --version`
	- Show `gdm` help message:
	`gdm --help`
	- Display help:
	`gdm --help`
	- Display version:
	`gdm --version`