March_24_Repairing_broken_links_in_Extend_section

[ArooshLele]

Describe your pull request here:
I added the missing syntax (.md) to the URLs in the files. This can solve the problem of the broken links. The syntax was added in the four files:

List the file(s) included in this PR:
cli-developing-a-plugin.md
cli-setting-up.md
mvd-plugindefandstruct.md
mvd-uribroker.md

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[github-actions]

🚀 Deployed on https://67ea80824a8e990bcb53754f--zowe-docs-master.netlify.app

[anaxceron]

thank you @ArooshLele

[ArooshLele]

thank you @ArooshLele