Docs updates for v1.5.0

[rkingsbury]

Various items in the docs can be updated for increased clarity. Specifically:

• charge_balance docstring - make explicit that this returns a SIGNED charge balance, positive or negative
• EOS.equilibrate: the base class docstrings should be updated to include the new kwargs added in v.1.4.0.
• Add a warning box to the "Quickstart" page alerting users that charge balancing and equilibration do not happen by default
• The doc pages headers say "pyEQL src documentation" ever since the migration to sphinx-immaterial (See migrate docs theme to sphinx-immaterial #272 ). Fix this to remove the "src" so that the pages just say "pyEQL documentation"
• Add a link to the GitHub releases / release notes page in the left navigation column

[rkingsbury]

Or consider migrating to Zensical to replace sphinx-immaterial