Add install instruction to package sidebar

[rl-king]

What

Copiable command line instruction that will install the package you're looking at.

Behaviour

• Triple click or drag to select text.
• Doesn't select > symbol.

Notes

• Tripple click always selects extra new line due to setting css overflow, not sure if or how this should be solved.
• Could be made more interactive by actually copying on click.

Preview

Fixes

#27

[rlefevre]

Install instructions have been added in the new "About" section of each package, for example:
https://package.elm-lang.org/packages/elm/file/latest/about

Note that triple click seems to work fine to select it (at least it does on Linux with Firefox for me).

[rl-king]

The current implementation also fits more text, which is an issue with having it in the sidebar.

Triple click selects a newline in Safari on macOs but not in Firefox, not sure if this is an issue worth fixing.