This documentation is not using the current rendering mechanism and is probably outdated. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
- At some point in the future, the TYPO3 extension manager (EM) is supposed to fully support Composer for dependency management. Right now, though, support for composer.json files don't seem very mature. The webmention feature depends on the jkphl/micrometa package (which isn't a native TYPO3 extension). Every now and then the EM reports a problem due to it's inability to find micrometa among the available TYPO3 extensions. The only workaround seems to remove the micrometa dependency from the composer.json file, clear the caches, re-run the desired operation and recreate the micrometa dependency afterwards. Another solution might be removing the composer.json file altogether (as long as Composer support in TYPO3 doesn't improve). The file will be recreated on extension updates though.