docs update for 1.0.0 and conversion from Sphinx ReST to MyST

[grinner]

Docs updated for new changes for primer3 2.6.1 migration

Migrated docs to MyST for sphinx Markdown.
All *.rst files are now *.md files

Update pass for README, CHANGES, quickstart, miscellany and index documentation for 1.0.0-staging branch

Updates to howtobuild.md and cython_help.md for better Markdown formatting

CHANGES and changes.md are now pre-prepended.

Sphinx Docs support type hinting via sphinx-autodoc-typehints

__version__ string in the __init__.py file is now the source of truth for all of the software. To be possibly updated with SCM migration

Made it consistent that primer3-py supports Python 3.8+ and not 3.7 or Python 2

Added a ReST include link to top level CHANGES file to get rid of duplicate file for maintenance that is, changes.rst is now changes_link.md

added .github/workflows/update-docs.yml to build docs when a new version tag is pushed to master branch

line length style updates for thermoanalysis.pyx