System functions: Standardise documentation page titles

DO NOT IMPLEMENT UNTIL #957 HAS BEEN COMPLETED

The system functions do not currently follow any naming conventions and some give a greater indication of purpose than others. A new set of names that should rectify these issues should be implemented.

When updating a system function name, it will need to be updated in the following places as well as in the title of its own page:

- the table on /language-reference-guide/system-functions/
- all mentions in tables on /language-reference-guide/system-functions/system-functions-by-category/
- the disambiguation page for the system function (if relevant)
- the ToC in the .yml file

As part of this exercise, the filenames should also be standardised. As well as changing the filename in \language-reference-guide\docs\system-functions, all cross-references will need to be updated. The filename convention to use is:

- monadic functions: foo-monadic.md
- dyadic functions: foo-dyadic.md
- disambiguation pages: foo-disambiguation.md

It is highly unlikely that system function names will be used in other places (usually a system function is just referenced as `⎕foo`), and unfortunately names have not always been stated consistently across the documentation set, so searching for individual references will be tricky and a low priority) – this can be ignored.

**NOTE**: When changing a filename, help_urls.h in SVN/docbin needs to be updated with the new filename. This must be committed at the same time as the other changes are merged as otherwise the build will break.

The attached spreadsheet includes the names that should be used.

[sysfn_names.xlsx](https://github.com/user-attachments/files/30539908/sysfn_names.xlsx)