The trick to using the terminal is getting good at googling issues. Fair warning that I have not used the MacOS command line or homebrew.

For the first issue, the first link to Googling "invalid active developer path" is this stackexchange post. The solution they provide is to run the command xcode-select --install.

For the second issue, the first link to Googling "Could not link: /usr/local/share/doc/homebrew" is this brew forum post. The solution they provide is to run the command rm -rf /usr/local/share/doc/homebrew (be careful because rm -rf is a dangerous command, make sure there's nothing in that folder you need).