feat: add export() method for PNG/SVG/PDF diagram output

[dharapandya85]

solves #21
this PR does:

• refractor display_network() and generate_gantt_chart() to extract figure-building logic into _build_network_figure() and _build_gantt_figure() private builders
• add export supporting .png, .svg, and .pdf
• PDF exports both charts as separate pages in one file

[dharapandya85]

@kathan-majithia , please review.

[kathan-majithia]

Great @dharapandya85, but why two gantt charts are exported ?

Also keep the logic simple, like what to export and in what format to export in export() or save() function.

[dharapandya85]

@kathan-majithia , removed gantt exports, and in export function, used ext variable which is checking for png, svg and pdf format, and one else-if chain, to save these formats in network or gant figures, what do you think about this, please share your opinion, would modify accordingly.