steamos-{session-select, readonly, dump-info, update}: add page

[Managor]

• 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):

[Managor]

This can be considered a draft. I was not able to find a documentation page for any of steamos-* commands.
Speaking of which, this can be also be considered a draft for all steamos-* commands.

[Managor]

This is probably ready, requesting review.

There is also steamosctl that's in development, but it's not out of its alpha phase yet.