Update i18n gettext tutorial to include POT generation, plurals and context support

[SkyLucilfer]

Update the gettext tutorial to incorporate changes from PR 39145 and PR 40443.

Since the second PR is not merged yet, the tr_n() function is still not yet defined.

However, EditorTranslationParserPlugin should normally be defined, since the first PR is already merged and I've written the class reference for it. I couldn't find EditorTranslationParserPlugin on Godot docs website either. Any idea why?

Here's the list of warnings I got when I built the html page.

D:\Game Dev\Godot\_engineDev\godot-docs\tutorials\i18n\localization_using_gettext.rst:76: WARNING: undefined label: class_object_method_tr_n (if the link has no caption the label must precede a section header)
D:\Game Dev\Godot\_engineDev\godot-docs\tutorials\i18n\localization_using_gettext.rst:238: WARNING: undefined label: class_object_method_tr_n (if the link has no caption the label must precede a section header)
D:\Game Dev\Godot\_engineDev\godot-docs\tutorials\i18n\localization_using_gettext.rst:270: WARNING: undefined label: class_editortranslationparserplugin (if the link has no caption the label must precede a section header)

Any feedback on the documentation is welcomed.

[SkyLucilfer]

@Calinou Do you have any feedback?
I have deleted one of the disadvantages where people have to install gettext tools. I feel like this shouldn't dissuade people from using PO files because installing Poeditor can be the solution to avoid using command-line. We open CSV using CSV application too.
Also I have moved the "Installing gettext tools" section to the end of the page.

[SkyLucilfer]

@Calinou Thanks for the review!