rename index automaton terminals

[kalmarek]

• rename isterminal → isaccepted
• swap accepted/non-accepted states, so that the language of IndexAutomaton consists precisely of words irreducible w.r.t. the underlying rewriting system.
• document IndexAutomaton
• document Settings
• fix some missing refs in docs

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.80%. Comparing base (3bef24c) to head (ccace0b).

Files	Patch %	Lines
src/Automata/interface.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #73   +/-   ##
=======================================
  Coverage   94.80%   94.80%           
=======================================
  Files          30       30           
  Lines        1829     1829           
=======================================
  Hits         1734     1734           
  Misses         95       95           

Flag	Coverage Δ
unittests	94.80% <95.00%> (ø)

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