Add 'dot2tex' under ExternalConditions

[ZachNewbery]

Why:

Closes #35

What's being changed (if available, include any code snippets, screenshots, or gifs):

Adding 'dot2tex' to ExternalConditions so that users are aware of it being required for digraph conversion.

Additionally, fixing CI badge link in README to conform to new format.

Check off the following:

• I have added the necessary functions to implement my fix in a structured, readable way.
• I have documented my added code to the codebase, added new directories to makedoc.g and new chapters/sections to order_info.g.
• I have tested my additions, and included the tests within the tst directory.
• I have summarised any large features as bullet points in CHANGELOG.md.

[olexandr-konovalov]

Looks good, although I am not sure why CI badge needs to be updated in the same commit PR.

[ZachNewbery]

Looks good, although I am not sure why CI badge needs to be updated in the same commit PR.

I noticed it was broken due to some changes to the URL formatting, didn't feel it was significant enough for a separate PR. I'll mention it in the description though for brevity!