✨ feat(header): add img_alt attribute for customisable alt text

[welpo]

Description

This PR introduces a new img_alt attribute within the header TOML configuration to allow for customisable alt text for the home banner image. Resolves #150.

Changes

• Added optional img_alt attribute to header variable for the main index page. Defaults to "the owner" (previous hardcoded value).
• Modified home_banner.html to use img_alt for the alt attribute if available.

Why is this change necessary?

The alt text for images is crucial for both SEO and accessibility. By enabling the img_alt attribute, we can provide a more descriptive and contextual alt text, which could vary based on the image used. This makes the theme more versatile and user-friendly.

How to test

1. Update the header in the TOML configuration of an _index.md file to include img_alt attribute like this:

   header = {title = "Hello! I'm Your Name.", img = "img/main.webp", img_alt = "Your Name, the site's author" }

2. Build the site and verify that the alt text has been updated as specified in img_alt.