* DOC: Simplify and update git setup page

Changed title from "Getting started with Git development"
to more specific "Setting up git for NumPy development"

Updated the outdated GitHub screenshot for forking button, added
more screenshots.

Simplified the narrative.

Fixed bad github links (always pointed to NumPy's github).

Clarified SSH setup, role, and effect on git remote -v.

Clarified meaning of "upstream" and "origin".

Replaced read of .git/config (platform-specific) with call to git config.

* DOC: Add links, remove section from PR #16671

Per suggestion, removed outdated advice on getting pull branches.

Added back rST anchors, should have left them in.

* DOC: Add page anchor for PR #16671

Add '.. _development-setup:' page anchor.

* DOC: Fixes for 18 broken links

This, with PR #16465, should fix nearly all the remaining broken links
on the site. 4 or 5 others should be easy to fix and just
need attention from someone more knowledgeable -- will
open an issue. For release notes with dead links,
I could usually find links on archive.org for roughly contemporary
versions.

* DOC: Update to "Fixes for 18 broken links #16472"

* Obsolete links, previously commented out, now deleted:
https://github.com/numpy/numpy/pull/16472#discussion_r433928958

* Semantic markup for reference to Python class:
https://github.com/numpy/numpy/pull/16472#discussion_r433553928

* Missing :ref: in internal link:
https://github.com/numpy/numpy/pull/16472#discussion_r433554484

Not included: Resolution on using external/internal doc link in .py:
https://github.com/numpy/numpy/pull/16472#discussion_r433554824

* DOC: Add internal link for 'Fixes for 18 broken links' PR #16472

Making reference [1] an internal link in function_base.py => numpy.vectorize.html

* DOC: Redirect 2 link fixes in PR #16472

* governance.rst link reverted
* ununcs.rst `overridden` link goes where it was meant to


per https://github.com/numpy/numpy/pull/16472#pullrequestreview-424666070

Fixed links for branch delete and graph visualizer.

Updated instructions for branch delete (git since 1.7.0 (Sept 2010)
has allowed --delete sted ':').

'Learn git' link pointed to Bitbucket instructions; replaced with GitHub
instructions.

Minor rewording in 'Pushing changes to the main repo'.

This is NOT a review of the page, only some fixes.

User- and non-user-facing typos.

Some source typos fixes as well.

Found via `codespell`.