docs: Misc formatting and text fixes, add some internal links

[digital-carver]

docs(Theory): Fix NFA state number errors, spelling, and add alt-texts to images

docs(Regex): fix "See also" links, minor punctuation and grammer fixes

docs(Home): minor grammar fix

docs(Validator): "See also" refs in _validator docstrings, improve readability

docs(Tokenizers): mainly formatting fixes, a couple grammar fixes too

docs: link to the simplified FASTA example when it's referred to

docs(Parsing buffers): clarify onfinal!, minor grammar fixes

docs(io,custom): minor grammar fixes, linkify reference to other section

[jakobnissen]

Thanks a lot! I'll merge this when CI is run in a bit

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (82a27f0) 95.92% compared to head (6874d8b) 96.15%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #132 +/- ## ========================================== + Coverage 95.92% 96.15% +0.22% ========================================== Files 16 16 Lines 1793 1793 ========================================== + Hits 1720 1724 +4 + Misses 73 69 -4 ``` | [Flag](https://app.codecov.io/gh/BioJulia/Automa.jl/pull/132/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/BioJulia/Automa.jl/pull/132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia) | `96.15% <ø> (+0.22%)` | :arrow_up: | 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=BioJulia#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/BioJulia/Automa.jl/pull/132?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia) | Coverage Δ | | |---|---|---| | [src/codegen.jl](https://app.codecov.io/gh/BioJulia/Automa.jl/pull/132?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL2NvZGVnZW4uamw=) | `92.83% <ø> (+1.14%)` | :arrow_up: | | [src/re.jl](https://app.codecov.io/gh/BioJulia/Automa.jl/pull/132?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL3JlLmps) | `93.40% <ø> (ø)` | | | [src/stream.jl](https://app.codecov.io/gh/BioJulia/Automa.jl/pull/132?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL3N0cmVhbS5qbA==) | `96.92% <ø> (ø)` | | | [src/tokenizer.jl](https://app.codecov.io/gh/BioJulia/Automa.jl/pull/132?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL3Rva2VuaXplci5qbA==) | `95.38% <ø> (ø)` | |

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