Adding Slack link

[d-spencer-strickland]

This adds a link to the C++ Slack workspace.

Tested under all breakpoints for various screen resolutions so it should render fairly well, but we can tweak it if there are any glitches.

[glenfe]

Hi Spencer, copying my reply to you on the Boost mailing list:

1. The place for these links (e.g. IRC and others) would be the Community page, not the main page.
   This: https://www.boost.org/community/index.html

2. Shouldn't this say "Visit the Boost channel at the C++ Slack" or something similar?
   i.e. You want to point people to the Boost space in a particular server.

[d-spencer-strickland]

@glenfe Thanks for the notes, I'll push an update by tomorrow to address. (sorry for the delay, want to keep this strictly RE: the work but have been dealing with a family emergency and want to note that since I hadn't had a chance to respond on this yet)

It seems like it will work best on the line that starts "If you are looking for more lively interaction..." and then also as a standalone link on the right nav either above or below the IRC link. If you have a suggestion on the ordering, please let me know. I am not sure which sees the most engagement from the community yet and will start following things more so that I'll have a better sense of things like that going forward. But will plan to push what makes sense to me now and can wait for any feedback. Thanks!