MAINT: migration updates (#36)

* MAINT: remove GitLab CI file

* MAINT: change URL

* MAINT: update URLs

* MAINT: update more urls

* MAINT: more URLs

* MAINT: change URL

* MAINT: remove /-/

* MAINT: remove /-/

* MAINT: add missing slash

* DOC: update URLs

* MAINT: fix pipeline status

* DOC: update contributing

* DOC: add confidential issue instructions

* DOC: tmp fix for milestone

* MAINT: update doc links

* MAINT: change branch to main

* Update README.rst

Co-authored-by: Colm Talbot <[email protected]>

* Update README.rst

Co-authored-by: Colm Talbot <[email protected]>

* Update README.rst

Co-authored-by: Colm Talbot <[email protected]>

* MAINT: change URLs

* MAINT: change URL

* DOC: update contributing

* DOC: change specific to confidential

* DOC: change gitlab references to github

* MAINT: update URLs for change from master to main

* MAINT: change merge request to pull request

* MAINT: capitalize Pull requests

* DOC: change pull main to fetch

* DOC: add note about migration to the changelog

* DOC: point to PR that updated the changelog

---------

Co-authored-by: Colm Talbot <[email protected]>