secret-tool: add page

[AntonFriberg]

Add page documenting simple commands to store and lookup secrets from
libsecret.

• [*] 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): libsecret-tools 0.21.4-1build3 (Ubuntu Noble)

[CLAassistant]

All committers have signed the CLA.

[tldr-bot]

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

pages/linux/secret-tool.md:5: TLDR004 Command descriptions should end in a period
pages/linux/secret-tool.md:22: TLDR008 File should contain no trailing whitespace

Please fix the error(s) and push again.

[tldr-bot]

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

pages/linux/secret-tool.md:5: TLDR004 Command descriptions should end in a period
pages/linux/secret-tool.md:22: TLDR008 File should contain no trailing whitespace

Please fix the error(s) and push again.

[tldr-bot]

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

pages/linux/secret-tool.md:22: TLDR008 File should contain no trailing whitespace

Please fix the error(s) and push again.