Fix HTML entities on the Digraphs homepage

[wilfwilson]

On the Digraphs homepage, https://digraphs.github.io/Digraphs, the entities &Digraphs; and ⪆ are being rendered as-is, not as we want them to be rendered:

This should be fixed, either by just having them as plain text, or by doing something else to make them special.

[forktree]

Apologies, I'm still working on this issue. My initial attempt borked the documentation page so I'm trying again, expanding the macro call.

Discussed with James at today's GAP meeting.

[wilfwilson]

The Semigroups package just deals with this issue by having the corresponding words in plain text:

I suggest we just do the same for Digraphs.

[wilfwilson]

Actually I came up with a better solution. See #809.