LtC updates for 2026 apr

[magpiedin]

Includes optional updates/suggestions:

• updates for the LtC site (as of April 2026)
  • fresh copies of LtC md, source csv's, and rebuilt html
• minor additions to .gitignore
  • for local VSCode cache and Mac index '.DS_Store' files
• suggested updates to the main Readme based on steps for ltc-standard working directory.
  • To do: generalize standard-specific utils scripts where possible

[magpiedin]

now includes French translation update from @ElieMarioSaliba - tdwg/ltc#123 (comment)