[#99] Remove link from `instance TypeHasDoc MText`

Description

Problem: that link got broken, looks like it does not worth referring outer sources in our autodoc at all.

Solution: write down rules for Michelson string values instead of providing a link to Michelson documentation.

Related issue(s)

Resolves #99 (closed)

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Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
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    • README
    • Haddock
    • docs/
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