Update user manual

[TransGirlCodes]

This PR is to address issue #140

Types of changes

This PR implements the following changes:
(Please tick any or all of the following that are applicable)

• ✨ New feature (A non-breaking change which adds functionality).
• 🐛 Bug fix (A non-breaking change, which fixes an issue).
• 💥 Breaking change (fix or feature that would cause existing functionality to change).

📋 Additional detail

• Add documentation section on Interfaces.
• Document Alphabets interface.
• Document BioSequence interface.
• Remove "not implemented" errors.
• Add iscomplete trait to Alphabets interface (see Add trait "iscomplete" to Alphabet #151).
• Make minimal examples unit tests, and or doctests.

☑️ Checklist

• 🎨 The changes implemented is consistent with the julia style guide.
• 📘 I have updated and added relevant docstrings, in a manner consistent with the documentation styleguide.
• 📘 I have added or updated relevant user and developer manuals/documentation in docs/src/.
• 🆗 There are unit tests that cover the code changes I have made.
• 🆗 The unit tests cover my code changes AND they pass.
• 📝 I have added an entry to the [UNRELEASED] section of the manually curated CHANGELOG.md file for this repository.
• 🆗 All changes should be compatible with the latest stable version of Julia.
• 💭 I have commented liberally for any complex pieces of internal code.

[codecov]

Codecov Report

Merging #160 (418886f) into v3 (9d40795) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##               v3     #160   +/-   ##
=======================================
  Coverage   82.14%   82.14%           
=======================================
  Files          31       31           
  Lines        2156     2156           
=======================================
  Hits         1771     1771           
  Misses        385      385           

Flag	Coverage Δ
unittests	82.14% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/alphabet.jl	81.66% <ø> (ø)
src/biosequence/biosequence.jl	91.42% <ø> (ø)
src/biosequence/predicates.jl	98.03% <ø> (ø)
src/longsequences/longsequence.jl	100.00% <ø> (ø)
src/longsequences/counting.jl	91.30% <100.00%> (ø)

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