Use the new re-entrant do-nothing script (#6062)

* Use the new re-entrant do-nothing script.

* Proper import error handling.

* Always use git add.

* More automation around CF standard names.

* Missing bracket.

* Less nebulous language.

* Encourage copy-pasting from previous releases.

* Clearer wording about release branches.

* Clearer RTD versioning.

* More precise installation tests.

* Clearer rc branch naming.

* Draw attention to deviation from conda-forge guidance.

* Wait for CI to finish.

* Include conda search command.

* Comment on discussion.

* Viva Engage.

* Use a git command for recreating files.

* Better SHA256 validation.

* Add missing conda deactivate command.

* Correct URL for watching conda-forge process.

* Correct use of self.git_tag.

* More guidance about when to change RC feedstock files.

* Better advice for waiting for conda testing.

* Raise correct message if `nothing` is not installed.

Co-authored-by: Elias <110238618+ESadek-MO@users.noreply.github.com>

* Line breaks in CF standard names instructions.

Co-authored-by: Elias <110238618+ESadek-MO@users.noreply.github.com>

* Don't use override-channels

* Shout louder about NOT targeting the main branch.

* More specificity in creating PRs, extra check for conda-forge RCs.

* Corrected sha256 check.

* What's New entry.

---------

Co-authored-by: Elias <110238618+ESadek-MO@users.noreply.github.com>

Author: trexfeathers

.gitignore

@@ -79,3 +79,6 @@ docs/iris_image_test_output/
 
 # Files generated during test runs.
 lib/iris/tests/results/**/*.dot
+
+# Uses of SciTools-incubator/nothing.
+**/.nothing

docs/src/whatsnew/latest.rst

@@ -84,6 +84,10 @@ This document explains the changes made to Iris for this release
    part of an ongoing effort to move from ``unittest`` to ``pytest``. (:pull:`6207`,
    part of :issue:`6212`)
 
+#. `@trexfeathers`_, `@ESadek-MO`_ and `@HGWright`_ heavily re-worked
+   :doc:`/developers_guide/release_do_nothing` to be more thorough and apply
+   lessons learned from recent releases. (:pull:`6062`)
+
 
 .. comment
     Whatsnew author names (@github name) in alphabetical order. Note that,