Rather than exact rules, you will find some guidelines for omitting articles in titles and headlines. More often, articles about writing a good title for a journal paper or article talk about conciseness in general. Knowing exactly which articles can be safely omitted can be tricky. I suggest finding published articles with as similar a subject matter as possible and using those titles as models.
Articles are usually omitted in titles and headlines to save space and
boost impact. How to Use Articles in Academic Writing
Articles are generally omitted in titles and headlines for conciseness
and directness. How To Use Articles In Academic Writing
The OP asks about the articles in this instance:
"The current example doesn't mention cases in which one of the
articles is dropped and another one is not. For example, a line from
the official Emacs reference card: "Save a file back to disk: C-x
C-s". Why the first article is kept, but the second one, before the
word "disk", is omitted?
The article is omitted here not to save space, but because the instruction is referring to a general category (all instances), rather than a specific instance.
WHEN NOT TO USE ARTICLES Non-native English writers commonly either
omit or overuse articles. Articles are not always necessary. Articles
are generally not used when referring to an entire category, such as
education, patience, music, etc. That is "Music enhances learning"
instead of "The music enhances learning." SciTechEdit
Paging, on the other hand, is the process of moving one or usually
a related cluster of pages from disk to memory (paging in) or from memory to disk ( paging out). ref
There is a large disparity between the time it takes to write
to memory and the time it takes to write to disk. ref
The former refers to processes moved from disk to memory, whereas
the latter corresponds to processes moved from memory to disk. ref
Read/reading from disk/memory and write/writing to disk/memory are the standard, idiomatic phrases used for computers.
In the first example above, the a related cluster of pages refers to a/any given cluster, whereas to memory and to disk are being conventionally used as a category without reference to any specific memory/disk. We see something similar with a Note/Memo to File used, for example, in a research trial to document something (e.g. conduct). We're interested in the documentation and not the specific physical file where the note is kept (usually a paper file in this case).
Note that processes has no article in the last example. These are processes in general.
In the case of a plural, ask yourself whether it is being used as a category or for specific instances. If you can replace the with these/those, it usually means the article is necessary:
Animals can be dangerous. (Animals in general; the category)
Be careful! The animals can be dangerous. (These animals... or the animals I was talking about.)
Cold water can make for an invigorating shower. Cold water in general.
The cold water was only one reason we gave the inn a bad review. Specific--the cold water (only) that came out of their shower.