Add other social media links in _config.yml

[anenadic]

Fixes #321.

Adding 3 new options in the _config.yml file:

theme_variables: 
  topnav:
    bluesky: false
    linkedin: false
    fosstodon: false

• bluesky: Enable or disable the appearance of the Bluesky nav link by adding the url towards the Bluesky page. Default: false
• linkedin: Enable or disable the appearance of the Linkedin nav link by adding the url towards the Linkedin page. Default: false
• fosstodon: Enable or disable the appearance of the Fosstodon nav link by adding the url towards the Fosstodon page. Default: false

[anenadic]

If this PR is accepted, the new config paramaters need describing in the documentation too @bedroesb.

[PhilReedData]

oh no, I've just done this same PR #325, albeit for Bluesky only. I did update the documentation though.

I checked for any duplicate work on Wednesday and didn't think to check again so soon.