Fix Sphinx warnings [ci skip]

[eric-wieser]

• g_inv and connect_flg were documented twice
• Mv constructor overloads needed :noindex: to avoid warnings
• The * in Grad was being treated as the start of emphasis (https://galgebra--409.org.readthedocs.build/en/409/generated/galgebra.mv.html#galgebra.mv.Mv.Grad)
• The main documentation needed a blank line.

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Bottom of https://readthedocs.org/projects/galgebra/builds/11127060/ should show the warnings. After this change, the only ones should be in README.md.