Improve gihub.io pages

[fpetrogalli]

After merging #65 , we need to:

• improve the rendering of the tables of intrinsics #81
• make sure that the CI and all contributors badges show up in the Home page.
• Add a button that says "report a bug" that points at https://github.com/ARM-software/acle/issues (done in [github.io] Add report bug and contribution buttons #86)
• Add a button that says "contribute with a proposal" that points at the top folder CONTRIBUTING.md (done in [github.io] Add report bug and contribution buttons #86 )
• remove Table of Contents from main README.md (done in [github.io] Correcting website formatting issues #82)
• In the navbar, change Date to Release date. (done in [github.io] Change navbar release date text #85)
• When clicking on sections in the TOC, it would be good if the section would not disappear under the nav bar. (Proposed solution: https://stackoverflow.com/questions/10732690/offsetting-an-html-anchor-to-adjust-for-fixed-header) (done in [gitHub.io] Adjust section anchors #94: https://arm-software.github.io/acle/mve_intrinsics/mve.html#extract-one-element-from-vector )
• Add a button to go back to the table of contents in the renderings of the specs (done in [github.io] Add back links to toc for section headers #118 )

[fpetrogalli]

We can close this issue as all the items in the task list have been addressed.