Assets: stead3-ru.pdf, stead3-en.pdf (modules3-ru.pdf, modules3-en.pdf) #50

As stead3.pdf is only in Russian now...

• https://github.com/instead-hub/instead/releases/download/3.x.x/stead3.pdf

... then need proper naming for this PDF's as:

• stead3-ru.pdf - for documentation in Russian;
• stead3-en.pdf - for documentation in English.

Also, as this PDF will be attached to releases, then links should look like:

• https://github.com/instead-hub/instead/releases/download/3.x.x/stead3-ru.pdf
• https://github.com/instead-hub/instead/releases/download/3.x.x/stead3-en.pdf

Same thing need also make for modules3.pdf:

• https://github.com/instead-hub/instead/releases/download/3.x.x/modules3-ru.pdf
• https://github.com/instead-hub/instead/releases/download/3.x.x/modules3-en.pdf

TODO

• English translation for "stead3" & "modules3" documentation #51
• Rename Russian docs "stead3.md" &"modules3.md" to "stead3-ru.md" & "modules3-ru.md" #52