Update CONTRIBUTING guidelines to e.g. refer to new default branch

[sadielbartholomew]

Change Summary

Whilst skim reading the contributing guidelines to put in the PR #673 I noticed it referred to the master branch but the default branch is named main so I guess it was renamed but that page has not been updated accordingly. There are also some links which I have updated:

• one which was not formatted properly to form a hyperlink, and also duplicated in terms of stating the URL, so I fixed it and removed the duplicate;
• one which now leads to a page which is marked in capitals at the top as a warning 'OBSOLETE CONTENT This wiki has been archived and the content is no longer updated. Please visit git-scm.com/doc for up-to-date documentation.' so I thought I should also update that to an up-to-date equivalent.

Related issue number

Trivial docs page set of minor updates hence I have not raised an Issue first.

Checklist

• [n/a] Unit tests for the changes exist
• [n/a] Tests pass on CI
• [n/a] Documentation reflects the changes where applicable

[mgrover1]

Sorry for the delay here @sadielbartholomew - taking a look at this today!

[mgrover1]

This looks great! Thank you for the contribution!