The same blog post you mention says:
The definite article the is used in front of any noun the listener or reader already knows about.
So, when you say:
Our teacher said that we need to use articles.
we now know about the teacher. Therefore:
The teacher talked about differences between two types of articles.
Is the natural way to say it, because the teacher was mentioned in the previous sentence. You wouldn’t use a instead of the, because you are talking about the teacher you already mentioned.
You could word something this way, though:
A teacher said that we need to use articles. That teacher talked about differences between two types of articles.
By the way, the “rule” you initially cited is trying to say that you don’t use the word teacher without any kind of modifier. In other words, you would NOT say this:
Teacher said that we need to use articles. Teacher talked about differences between two types of articles. (incorrect)
We don’t generally begin sentences with words like teacher, carpenter, doctor, or policewoman. Instead, we use article, possessive pronouns, or some other determiner:
Our teacher said...
A carpenter told me...
My doctor wants...
That policewoman saw...
Best Answer
There's not actually a single answer to this... it depends on the company structure, which you haven't included in your question, but that's ok.
Here's your sentence:
If you were the only person at that company with that job title, the correct article is "the".
Now, if you were one of two or more people with this position, then you would use the indefinite article. Here, the choice is between "a" and "an". The rule is
In your case, e-shop is a vowel sound, so you'd go with "an".
You can use neither article, though, if you wish. Because this position is sort of your "title", you could certainly opt to exclude them.
If it helps, you can imagine "e-shop back-end and front-end software developer" is in quotes but don't include the quotes on your resume/CV.
As an added note, in the versions with articles, you don't necessarily need the word "position":
This version is perfectly fine and (to my ears) sounds slightly better.