doc: fix error msgs, add troubleshoot entry and instructions when creating a new endpoint keyword

  - In `doc/analysis/requests.py` if it is an endpoint operation (in `self.op` with the list and position of parameters,

    in `self.description` to reference the method that pretty print the operation, and add the method that pretty prints

    the operation)

- When a new endpoint Keyword is created, additions have to be done in `doc/analysis/requests.py`:

  - Add corresponding entries in the `self.op` and `self.description` dictionaries in the initialization function of the `Requests` class

  - Implement the function generating the documentation related to the endpoint Keyword 

- When a new permutation is added in an existing Test Case, run the documentation generation script 

  (`python doc/generateDocumentationData.py {tc_id}`) for the Test Case and copy the generated JSON file in the 

  folder containing all files for the given group and subgroup (`cp doc/results/{tc_id}.json doc/files/{group}/{subgroup}`)

If a Test Case is built around permutations, you have to use the `Test Setup` (and `Test Teardown`) keywords in order

to properly generate the documentation. If you use `Suite Setup` (and `Suite Teardown`), the generated documentation

will not contain the full description of the setup step (it will not full but it will only contain the generic sentence).

will not contain the full description of the setup step (it will not fail, but it will only contain the generic sentence).

## SyntaxWarning: invalid escape sequence '\s'

When generating the documentation, such a warning may be displayed: 

```shell

/some/path/ngsi-ld-test-suite/doc/analysis/requests.py:463: SyntaxWarning: invalid escape sequence '\s'

  regex = '(\s{4})*\s{4}\.{3}\s{4}(.*)'

```

It usually means you are calling a keyword without using named arguments (note that this does not happen for all the

keywords).

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