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 :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #73 +/- ## ======================================= Coverage 94.80% 94.80% ======================================= Files 30 30 Lines 1829 1829 ======================================= Hits 1734 1734 Misses 95 95 ``` | [Flag](https://app.codecov.io/gh/kalmarek/KnuthBendix.jl/pull/73/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marek+Kaluba) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/kalmarek/KnuthBendix.jl/pull/73/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marek+Kaluba) | `94.80% <95.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marek+Kaluba#carryforward-flags-in-the-pull-request-comment) to find out more.

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