Documentation rework

This issue describes future task in order to improve the documentation of IoT Agents. After a initial analysis, the current files need to be reworked and merged into different files. The desired file structure, contining previous documentation is the following:

The implementation plan is creating a branch (task/doc-rework) with all the documentation changes and merge by creating a PR containing all the changes. Previous smaller PR, one for each file (api.md, admin.md ...) should be opened against task/doc-rework to ease the review.

After reworking files, some tasks should be acomplished:

• Sync with master https://github.com/telefonicaid/iotagent-node-lib/pull/1436
• Review the whole documentation in order to remove deprecated / removed features. Things to check (non exhaustive list) https://github.com/telefonicaid/iotagent-node-lib/pull/1437:
  • References to "plugin" (and its variant "plug-in")
  • References to "bidirectional" (deprecated feature)
  • References to "appendMode" and "IOTA_APPEND_MODE" (and its variants "append" and "APPEND") (deprecated feature)
  • References to "legacy" and " expressionLanguage" (legacy expressions)
    Deprecate: appendMode
• Decide and rename if necessary remaining files name and places https://github.com/telefonicaid/iotagent-node-lib/pull/1446
  • Add readme.md under doc folder
  • update index.md
  • update RTD .yaml

Probably, at this point, merging task/doc-rework with master should be considered, since the rest of the takes could take much more time. Then:

At this point, iotagent-node-lib 3.4.0 has been released. The remainer tasks would be done in a next version

• https://github.com/telefonicaid/iotagent-node-lib/pull/1470
• #1632
• #1645
• https://github.com/telefonicaid/iotagent-node-lib/issues/1409
• https://github.com/telefonicaid/iotagent-node-lib/issues/1345
• https://github.com/telefonicaid/iotagent-node-lib/issues/1372
• Add binary data support doc (commands, measurements, specific paths, headers...)
• Final check to improve advanced features - specially related with JEXL features
  • Add a list where expression are accepted (I.E: entityNameExpression, explicitAttrs, endpoint...)
  • Include skipAttribute examples
  • Include entityNameExpression
• Review warnings included due to potentialy outdated documentation:
  • doc/devel/development.md#db-models (interesting)
  • doc/devel/development.md#function-reference (hard)
  • doc/devel/development.md#developing-a-new-iot-agent (hard)