Tiny accessibility fixes

[manuhabitela]

👋

Those are micro changes to better match our accessibility standards:

• add the website context in the title
• change link color to match color contrast ratio required by RGAA and underline links to mark the difference with surrounding text. I chose DSFR blue
• add an alt text for the login button, for screen readers
• wrap everything in a <main> tag as nothing should be outside region tags in theory

I know it's not that necessary on those pages but now they can pass automated a11y tests if we setup any (see numerique-gouv/moncomptepro#508) :)

@rdubigny @douglasduteil