Create ISTG cover art and align on look/feel with MSTG where possible

[scriptingxss]

MSTG is a mature flagship OWASP project with a large following and a steady flow of contributors maintaining the guide. Until ISTG has a similar maturity and following, it'll be challenging to keep up with MSTG. Although, we should aim to align where it make sense and build relationships with testing guide project leaders for support.

Observations and opportunities to align

• ISTG should have a similar cover to MSTG's
• GitHub pages theme (material) should be similar
• Add a download link to the checklist and other formats like PDF
• Input validation category abbreviation detailed in https://github.com/OWASP/owasp-istg/issues/4

[scriptingxss]

• Cover art created ✅ https://github.com/OWASP/owasp-istg/commit/7ad853f958f30df5d556aa5a967440b41bb0e239#diff-819b1ec85f8148cbf5b271d1026f4cc4d8603730b8c3b7f6549bc7b87e974e6d
• Adopted portions of MSTGs contributing page linked in the readme ✅ https://owasp.org/www-project-iot-security-testing-guide/#div-contributing
• Input validation category abbreviation updated along with the component overview ✅

Open items

• GitHub pages theme (material) should be similar - Had a sync with Carlos from MSTG and walked through how they generate their website 🔜
• Add a download link to the checklist and other formats like PDF as github action - in progress but we have markdown and excel available. Official release will have downloadable links 🔜