feature: Load help docs from local plugins

[myarcana]

Did you check the docs?

• [X] I have read all the lazy.nvim docs

Is your feature request related to a problem? Please describe.

As soon as I cloned tpope/vim-eunuch locally into my lazy dev directory and added dev = true to the plugin spec

{
    'tpope/vim-eunuch',
    dev = true,
},

:help eunuch, :help :Unlink etc. stopped working, and vim-eunuch is not listed in :help local-additions anymore.

In :Lazy vim-eunuch now looks like:

    ● vim-eunuch 2.67ms  start
        dir    /Users/me/vim-eunuch
        url    https://github.com/tpope/vim-eunuch
        branch master
        commit f20a56b
        help   |eunuch.txt|

Trying to CTRL-] on |eunuch.txt| errors

E433: No tags file                                                                                                                                                                                     
E426: Tag not found: eunuch

Describe the solution you'd like

I think local plugins should have their help docs imported too.

Describe alternatives you've considered

I cannot think of any alternatives.

Additional context

No response

[folke]

Do an update check. That should generate the help docs for dev plugins (if you're on lazy main)