fixes to headers and toc and internal links

[tlienart]

• fixes issues with header links for relative pages
• also fixes the automatically generated table of contents
• fixes tests accordingly

[codecov]

Codecov Report

Merging #224 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #224     +/-   ##
=========================================
+ Coverage    87.9%   88.01%   +0.1%     
=========================================
  Files          28       28             
  Lines        1207     1218     +11     
=========================================
+ Hits         1061     1072     +11     
  Misses        146      146

Impacted Files	Coverage Δ
src/parser/md_chars.jl	100% <ø> (ø)	⬆️
src/parser/tokens.jl	100% <ø> (ø)	⬆️
src/JuDoc.jl	100% <ø> (ø)	⬆️
src/converter/md.jl	98.14% <ø> (ø)	⬆️
src/converter/lx.jl	95.57% <ø> (ø)	⬆️
src/converter/html.jl	95.83% <ø> (ø)	⬆️
src/parser/html_tokens.jl	100% <ø> (ø)	⬆️
src/manager/file_utils.jl	82.69% <100%> (ø)	⬆️
src/converter/html_functions.jl	92% <100%> (ø)	⬆️
src/misc_html.jl	100% <100%> (ø)	⬆️
... and 4 more

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