nh: add page

[luskai]

Adds a new TLDR page for the nh (Nix Helper) command, with brief summary and examples.

Closes #17495

[CLAassistant]

All committers have signed the CLA.

[tldr-bot]

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

pages/common/nh.md:
Error: Parse error on line 18:
...ments home-manager- nh darwin - implim
---------------------^
Expecting 'BACKTICK', got 'DASH'
pages/common/nh.md:7: TLDR015 Example descriptions should start with a capital letter
pages/common/nh.md:7: TLDR005 Example descriptions should end in a colon with no trailing characters
pages/common/nh.md:8: TLDR007 Example descriptions should be surrounded by empty lines
pages/common/nh.md:10: TLDR015 Example descriptions should start with a capital letter
pages/common/nh.md:10: TLDR005 Example descriptions should end in a colon with no trailing characters
pages/common/nh.md:11: TLDR007 Example descriptions should be surrounded by empty lines
pages/common/nh.md:13: TLDR015 Example descriptions should start with a capital letter
pages/common/nh.md:13: TLDR005 Example descriptions should end in a colon with no trailing characters
pages/common/nh.md:14: TLDR007 Example descriptions should be surrounded by empty lines
pages/common/nh.md:16: TLDR015 Example descriptions should start with a capital letter
pages/common/nh.md:16: TLDR005 Example descriptions should end in a colon with no trailing characters

Please fix the error(s) and push again.

[Managor]

@luskai Fix the bot issues.