emphasize need to have dot (graphviz) installed #7225

[pdurbin]

What this PR does / why we need it:

Contributors trying to help with documentation may be confused by this cryptic warning (which we treat as fatal as of pull request #7093):

Warning, treated as error:
WARNING: dot command 'dot' cannot be run (needed for graphviz output), check the graphviz_dot setting

make: *** [html] Error 1

Which issue(s) this PR closes:

Closes #7225

Special notes for your reviewer:

The whole "Writing Documentation" page could use a re-write but I just made a couple small edits.

Suggestions on how to test this:

Try setting up a new machine to build the guides.

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

No.

Is there a release notes update needed for this change?:

No.

Additional documentation:

None.

[coveralls]

Coverage remained the same at 19.535% when pulling 79825db on 7225-graphviz into 796e612 on develop.

[mheppler]

👍 Looks good.