typecode table should appear right below the identifier page

[turbomam]

change mkdocs.yml to

nav:
  - Home: home.md
  - NMDC Schema: index.md
  - NMDC Metadata Standards Documentation: Metadata_Documentation_Overview.md
  - NMDC Schema Validation: schema-validation.md
  - Identifiers in NMDC: identifiers.md
  - Table of Identifier Typecodes for each Class: typecode_table.md
  - MIxS Soil Package: NMDC_MIxS_Soil_documentation.md
  - Credits: credits.md

[turbomam]

semi-related: add comment to typecode table about needing criteria for making good typecodes

[turbomam]

see also

• https://github.com/microbiomedata/nmdc-schema/issues/599