docs: simplify a link in the html front page

index.html and gcode.html live in the same directory: the root of the
generated html tree.  So there's no need to bounce around directories
when linking.
docs: clarfiy G53

"absolute coordinates" might be confused with G90, calling it the "machine
coordinate system" seems clearer.
docs: remove a bogus entry in the M-code TOC

Looks like a silly cut-n-paste error.
docs: fix the description of M60

The M-codes document uses to have the wrong title for the M60 section.

It used to say "Program Pause", but it's really "Pallet Change Pause"
according to the gcode overview document and according to the actual
full description of M60.
docs: make T in the gcode overview point to T instead of M6

But include a link to M6, for completeness....
docs: fix a typo in the M48, M49 link text

The links to M48 & M49 were broken, this commit fixes them.

M48 and M49 are in the same section, so combine their entry in the TOC.
docs: add missing type attribute for <script> element

The <script> element requires a type attribute, which identifies the
type of script present.

Problem identified by http://validator.w3.org.
docs: add missing charset

Valid HTML documents specify the charset used.

Problem identified by http://validator.w3.org.
docs: "/" is not allowed in html anchors

asciidoc anchor inline macros render to html anchors, but "/" is not an
allowed character in html anchors.

This anchor is never actually used by our docs, so changing it doesn't
require changing any links.

Problem identified by http://validator.w3.org.
docs: " " is not allowed in html anchors

asciidoc footnotes become html anchors, and the " " character is not
allowed.

Problem identified by http://validator.w3.org.
docs: split the G43/G43.1 anchor

This makes individual anchors for G43 and G43.1, instead of having a
single combined anchor for both, pointing to G43.

Also replaces the "." in the G43.1 anchor name with "_", since "." is
not a valid anchor character.  There are no links to the G43.1 anchor,
so none need to be changed to the new anchor id.
docs: split the G61/G61.1/G64 anchor

This commit creates separate anchors for G61/G61.1 and G64, instead of
having a single combined anchor for all three (pointing at G61/G61.1).

Also makes the G61/G61.1 and G64 anchors into proper BlockId elements.
This includes replacing the "." (invalid) in the anchor name with "-"
(valid).
docs: fix the links to M50/M51/M52/M53

In the gcode overview document, all 6 of these M-codes used to be lumped
together and share a link, which pointed at M48/M49.  This commit uses
the pre-existing separate anchors for M48/M49, M50, M51, M52, and M53,
which seems clearer.

Also changes the asciidoc anchor inline macros to valid BlockId elements,
which makes for clearer HTML output.

And fix a broken link in the M-code document TOC.
docs: split the G92/G92.1/G92.2/G92.3 anchors

The G92, G92.1/G92.2, and G93 G-codes already had their own separate
anchors, this commit makes the gcode overview document link to the proper
one instead of all linking to the G92 anchor.

Replace the asciidoc anchor inline macros with proper BlockId elements.

Replace "." (invalid) in anchors with "_" (valid).
docs: create proper asciidoc BlockId elements

Having the [[anchor-name]] after the asciidoc Title element creates an
anchor inline macro that points just after the title text, which renders
incorrectly on some browsers.  Clicking the link would take you to just
after the title text, which is disorienting.  Plus, the HTML just looks
wonky, with an empty anchor element inside the h2 element body.

This commit replaces the anchor inline macros with proper asciidoc BlockId
elements preceeding the title text, which produces sane HTML: the title
element itself has an id attribute with the anchor text in it.  This
displays correctly on all tested browsers, and just generally seems
reasonable.

Also replace "." (invalid) in anchors with "_" (valid).

For reference: http://www.methods.co.nz/asciidoc/userguide.html#X41
docs: fix M62-M65 links

The link to M62-M65 in the m-code document TOC was broken (the link in
the gcode overview document was good).

Also change the link text from "M62-65" to the format used throughout
the rest of the documentation: "M62...M65".
docs: fix a broken link in the stepper quickstart

The link to the stepconf wizard was misspellt.
docs: fix a broken link in the HAL intro

Looks like you can't put BlockID elements on image blocks.  Move the anchor
to the preceeding paragraph that describes the figure in the image.
docs: fix a broken link in the User Concepts doc

The cutter comp section was renamed from "sec:cutter-radius-compensation"
to just "sec:cutter-compensation" back in May, and this link wasn't
updated.
docs: fix links in glade gui docs