Description

Add links to the ZSH Plugin Standard to Writing_Plugins_and_Themes.md.

Type of changes

[x] This document is covered by the BSD License, and I agree to contribute this PR under the terms of the license. This is for the list submission, not for the project(s) you're adding, I don't care what license the plugins have as long as they have something.

[x] I have read the CONTRIBUTING document.
[x] All new and existing tests passed.
[x] I have confirmed that the link(s) in my PR is valid.
[x] I have signed off my commits. You can use git commit --amend --no-edit --signoff to amend an existing commit, and you can find more details about signing off commits on the DCO GitHub action page here.
[ ] My entries are single lines and are in the appropriate (plugins, themes, or completions) section, and in alphabetical order in their section.
[ ] The completion/plugin/theme has a plugin file in the repository that conforms to the ZSH Plugin Standard - TLDR, there's a plugin file with a .plugin.zsh, .zsh or .sh suffix, it is not just bare instructions to be added to .zshrc
[ ] Any added completions have a readme and a license file in their repository.
[ ] Any added frameworks have a readme and a license file in their repository.
[ ] Any added plugins have a readme and a license file in their repository.
[ ] Any added themes have a screenshot, a readme, and a license file in their repository.
[ ] I have stripped any leading and/or trailing zsh-, zsh-plugin and/or oh-my-zsh- substrings from the link's displayed name. This makes it easier to find plugins/themes/completions by name by preventing big clusters in the O and Z sections of the list.