Update copyright year to 2023.

Rename all occurences of SIRIL to Siril.

Use hash-style formatting for headings.

Use dashes for unordered lists.

Separate binary downloads from build instructions. Remove Debian
package from list of binaries as we no longer provide that.

Simplify and/or fix git instructions.

Simplify and/or fix meson instructions.

Use backtick-style for code blocks and add language indicator for
syntax highlighting.

Add a few more links where appropriate.

Add a license section with SPDX-style identifier.

Make markdownlint happy. See rules here:
https://github.com/DavidAnson/vscode-markdownlint#rules