…electable text should be excluded from copy and do so if needed. [_static/copybutton_funcs.js] Added functionality to get unselectable elements and remove their text.

…ith funcitons using .innerText. [package.json] Added browser-env module for DOM support in testing. [docs/index] Updated documentation. [copybutton, copybutton_funcs] Fixed minor errors. Updated default exclude text. [__init__] Updated default exclude text.

for more information, see https://pre-commit.ci

- `.linenos` is the line numbers, and `.gp` is the tag from the
  pygments highlighter for things like the `$` prompt.  Upstream
  sphinx CSS excludes `.gp` from being copied, see the analysis here:

  executablebooks#138 (comment)

- If upstream sphinx excludes `.gp`, we probably should as well.

- Re-add documentation from PR executablebooks#138 - that PR got conflicted with a
  docs rearrangement, so I add the docs back in one chunk now.  I
  include my own suggestions in that PR.